Chapter 6: Toxicology

Toxicology comprises several medical applications. The analysis of drugs in human specimens can be conducted for clinical or legal/forensic purposes. Clinical applications include the acute management of overdose and therapeutic monitoring of drug concentrations to achieve maximum efficacy while limiting the toxicity and side effects of medications. Forensic applications of toxicology include analysis of drugs to provide evidence for civil and criminal court cases, to investigate the cause of death, to deter the use of performance-enhancing drugs in athletic competitions, and to determine operator impairment related to traffic citations and vehicle accidents. Workplace drug testing assesses pre-employment drug abuse and on-the-job impairment. Given the number of therapeutic drugs, drugs of abuse, and environmental toxins, as well as the variety of diseases, signs, and symptoms associated with drug exposure and overdose, there is an array of laboratory testing strategies. For this reason, the discussion of toxicology in this chapter will be divided into 3 broad sections—therapeutic drug monitoring (TDM), drugs of abuse, and environmental toxins (Figure 6–1).

Overview of Therapeutic Drug Monitoring

TDM is the practice of measuring the concentration of a drug or its metabolite in order to optimize the dosing of that drug to an individual patient and/or to assess patient compliance with a dosing schedule. The goal of TDM is to improve drug efficacy—the likelihood of a therapeutic effect while avoiding or minimizing adverse effects. Table 6–1 lists some commonly monitored drugs. Patients do not require monitoring for most drugs. However, for a limited group of agents or for patients with certain conditions (for instance, limited renal function, pregnancy, newborn or geriatric age groups), TDM plays an essential role in establishing the appropriate therapeutic dosing regimen.

Prior to the 1960s, drug dosing was entirely empirical. For certain agents, this trial-and-error approach gave wide variations in patient response and a significant incidence of toxicity. Since then, physicians have learned to optimize drug dosages and delivery while avoiding many of the drug's adverse effects. This has been achieved through the development of sensitive and rapid laboratory assays and the establishment of therapeutic ranges for common medications.

Indications for Therapeutic Drug Monitoring

TDM is performed to optimize the dose of a drug to an individual patient. Drugs with a narrow therapeutic index or margin of safety (the difference between the effective dose and the toxic dose) are potential candidates for therapeutic monitoring (Table 6–2). TDM is useful for drugs that display significant pharmacokinetic variability that may be caused by drug interactions, genetic variation in drug metabolism, nonlinear kinetics, physiologic conditions such as pregnancy and aging, and underlying diseases that alter the effective amount of drug delivered to or metabolized by the body. When patient compliance is in question, drug monitoring may be used to demonstrate the presence or absence of the prescribed agent. TDM requires a suitable laboratory assay and the establishment of a therapeutic reference range that correlates with efficacy and/or toxicity.

TDM is performed by measuring the concentration of a drug and metabolite(s). Blood or serum/plasma is the usual sample for TDM, but in some cases, urine or oral fluid samples are used to evaluate patient compliance. The most common examples of urine and oral fluid sampling are monitoring of buprenorphine, methadone, and oxycodone for compliance. By using blood levels to guide drug therapy, a proportional relationship is assumed between the plasma/serum concentration, the concentration of drug at the organ cellular level, and pharmacologic effect. For practical reasons, only blood levels of the drug are measured, because tissue concentrations cannot be easily sampled or analyzed. This pharmacokinetic principle of homogeneity defines the timing of sampling for TDM, since the concentrations of drug in blood at the moment of sample collection must reflect a proportional and constant (steady-state) concentration at the end organ and be reflective of drug effects at the cellular level. Most TDM samples are collected as trough concentrations, the lowest level just prior to the next dose, or as peak concentrations, 30 to 60 minutes after the dose, when blood levels are most reflective of the tissue concentration and drug efficacy or toxicity.

Pharmacokinetic Principles

Pharmacokinetics is the study of drug interaction (absorption, metabolism, and clearance) within the body. Drug behavior in the body can be described by the LADME mnemonic. The “L” stands for liberation or release of the drug from its dosage form. The “A” is absorption that describes the movement of drug from the administration site into circulation. Distribution is the “D” and describes the reversible movement of drug through the circulatory system and body tissues. Metabolism or “M” is the chemical conversion of drug to active and inactive compounds. Finally, the “E” indicates how the body eliminates the drug.

Drugs behave in the body based on their chemical characteristics at the molecular level. Drugs can be acidic, basic, neutral, or polar (Table 6–3). The charge and dissociation constant (pK) of the drug influences its absorption, distribution, and elimination characteristics. The dissociation constant of the drug also affects how the drug can be extracted from patient samples and analyzed in the laboratory.

Liberation and Routes of Drug Administration

Drugs can be delivered to the body in a variety of ways. Patients may take a drug orally (PO) by pill (eg, aspirin) or dissolve a powder in a liquid drink (eg, laxatives). Drugs can be delivered intravascularly (IV) through a needle directly into circulation (antibiotics, like vancomycin). Some medications are delivered under the tongue, sublingual (SL), like nitroglycerin for cardiac pain or angina. Others may be injected under the skin, subcutaneous (SC), like compounds in a tuberculosis test, or intramuscularly (IM), such as vaccinations. Rectal (suppositories) and transdermal (eg, fentanyl pain patches) are other methods of delivering a drug. The route of administration will affect absorption and bioavailability of the drug to the body.

Drug Absorption

The route of drug administration and the formulation of the drug affect the rate and extent of drug absorption. For example, oral drug absorption is affected by many factors including drug solubility in enteral fluid, the acid–base characteristics of the drug, the lipid solubility of the drug, interferences with absorption by food, destruction of the drug by gastrointestinal flora, coadministration of other drugs—especially antacids, cholestyramine, and other resin-binding agents—blood flow to the gastrointestinal tract, and gastrointestinal transit time. Some orally delivered drugs are also subject to a significant “first-pass effect,” whereby they are largely metabolized by the liver to inactive compounds before reaching the systemic circulation. IV administration delivers drug directly into circulation bypassing “first-pass metabolism,” and the amount of drug delivered IV is often compared with other delivery options for determining the extent or amount of drug that is absorbed from a specific formulation. Significant variability in drug absorption is thus a common indication for TDM.

The chemical characteristics of a drug also affect the rate and extent of drug absorption. Acidic drugs carry a carboxyl group, R−COOH (Table 6–3). This acidic group is unionized or uncharged at pH levels below the drug's dissociation constant and is ionized or charged (R−COO⁻) at pH levels above the drug's dissociation constant. Drugs that act as strong acids have dissociation constants with a pK <5, such as salicylate, penicillin, and analgesics. Strongly acidic drugs are unionized and do not carry a charge at the acidic pH of the stomach while they carry a charge at the more basic pH of the intestines. So, drugs like salicylate are passively absorbed in the stomach, but require active transport for absorption across the intestines. Weak acids (barbiturates, sulfonamides, and thiazide diuretics) have dissociation constants in the range 5 to 11, and are preferentially absorbed in the intestines compared with the stomach. Strongly acidic drugs tend to be fully dissociated or charged at blood pH of 7.4 while weak acids may have significant amounts of the unionized form present in the blood. Due to the acidic pH of urine, weak acids that are ionized at blood pH may become unionized in urine and prone to greater reabsorption. Basic drugs contain an amine group (R−NH₃). Basic groups are ionized (R−NH₂⁺) below and unionized (uncharged) above their dissociation constant. Basic drugs can act as weak bases (eg, anesthetics, opiates, and antidepressants) with pK <10 or strong bases (eg, amphetamines and bronchodilators). Basic drugs tend to be significantly ionized (charged) at blood pH. Drugs can also be neutral and carry no charge across the range of physiologic pH. Neutral drugs can be lipophilic and act like fats (eg, corticosteroids) or polar and hydrophilic and attract water molecules (eg, digoxin).

Distribution

After absorption, drugs distribute throughout the body through the circulatory system, lymphatic system, and tissue fluids. The amount of free drug available to act at organ receptors is affected by both protein and tissue binding. Protein binding is another consideration in TDM. Binding to plasma proteins occurs to some extent for most drugs, with bound and free (unbound) drugs existing in equilibrium. Although it is only the free drug fraction that is biologically active, most laboratory assays measure the total drug concentration, that is, the sum of the bound and the unbound drugs. Several factors can cause changes in plasma proteins and, consequently, affect free drug levels. Acid/neutral drugs tend to bind albumin while basic/neutral drugs bind α₁-acid glycoprotein. Some drugs have specific binding proteins such as cortisol and corticosteroid-binding globulin, also known as transcortin. These proteins serve as transport proteins for drugs from the site of absorption to the tissue where drug can act, and as delivery mechanisms to the liver for metabolism or the kidney for elimination. Disease alterations in protein concentration can affect the concentration of free drug. For example, hypoalbuminemia, which occurs in the elderly and in patients with cirrhosis, may cause an increased free drug fraction in the setting of normal total drug levels. α₁-Acid glycoprotein is an acute-phase reactant and levels of this protein increase in acute and chronic diseases. Increases in α₁-acid glycoprotein create more binding sites for drug, so less free drug will be available in light of the same total drug concentration in a sample. The presence of uremia in disease results in compounds binding to albumin, displacing drug from the protein and elevating the free drug fraction. TDM can determine the proportion of free and total drugs in disease and individualize the dosage to the patient's condition.

Drug Metabolism

Drug metabolism typically renders nonpolar, lipophilic drugs into more polar, water-soluble compounds for elimination. The liver is the primary site for drug metabolism. Genetic variants, age, cirrhosis, and other hepatic conditions may adversely affect drug metabolism, and thus predispose a patient to toxicity. Many drugs are hepatic enzyme inducers or inhibitors and thus can influence the rate of their own metabolism, as well as the metabolism of many other drugs. Pharmacogenetics is study of drug action and metabolism based on genetics. Different genes can lead to changes in drug metabolism and produce an individualized response to a drug. Fast metabolizers will change parent drug to a metabolite at a greater rate than slow metabolizers, depending on the genes expressing metabolizing enzymes in the patient. For example, patients with the slow metabolizer gene will acetylate procainamide (a cardiac drug) to N-acetylprocainamide metabolite at a slower rate than fast metabolizers, and may be more prone to toxicity while on the same dose of drug.

Drug Elimination

Elimination is the removal of drug and metabolites from the body. Drug can be eliminated through the kidneys, the liver, the lungs, the skin, the feces, and by other means. Elimination of many polar, nonlipophilic drugs is achieved primarily through renal excretion, which is dependent on adequate kidney function and renal blood flow. Other parameters relevant to elimination through the kidneys include urine pH and the properties of the drug itself, such as the dissociation constant, pK, and molecular size. Drug clearance is the theoretical volume of serum/plasma that is completely cleared of a drug per unit time. Importantly, clearance is the sum of all elimination mechanisms—hepatic, renal, lung, and any other—for a particular drug. Patients with impaired drug clearance may need more frequent monitoring.

In TDM, drug levels are most often determined only after steady state has been achieved. Steady state is the condition that occurs when the amount of drug entering the system equals the amount being eliminated. Steady-state concentration is compared in relation to a target range to determine changes in dosing. The target range is established from experimental dosing studies to determine the optimum drug concentration where a drug is most effective while causing the least undesirable side effects and toxicity. The target range is a generalized range that fits most patients, but that range may need to be adjusted or altered in certain disease states and physiologic conditions. TDM allows physicians to optimize drug dosage to a patient's individual situation.

Most but not all drugs are eliminated by first-order (or linear) kinetics. This means that a constant fraction of total drug is eliminated per unit time. All drugs have a biological half-life. For drugs that follow first-order elimination kinetics, changes in dose will generally cause predictable changes in blood levels. Increases in drug concentration lead to increases in the rate of drug elimination. Some drugs, however, are eliminated by zero-order (or nonlinear) kinetics, such that a constant amount of drug is eliminated per unit time. Typically, metabolism by zero-order kinetics occurs when elimination pathways for that drug have been saturated. Under these circumstances, the biological half-life is not constant but depends on drug concentration. As a result, small increments in dose may cause disproportionately large increments in blood levels. Due to their lack of a predictable dose–response relationship, drugs that follow zero-order kinetics often require monitoring.

Assuming first-order kinetics, 5 half-lives are required after initiation of drug therapy to reach nearly complete (97%) steady state (5 half-life rule). Five half-lives are also required for nearly complete clearance of a drug after the termination of therapy, and for attaining a new steady state whenever a dosing regimen has been changed.

Drug Interactions and Dose Adjustments

Many patients may take more than 1 medication, and those drugs can interact in the patient's body. Drug interactions may cause displacement of bound drug from proteins. The clinical significance of the interaction is likely to be increased when both drugs are highly protein bound (80% or more), when 1 of the drugs has a higher binding affinity, or when 1 of the drugs is present in higher concentration than the other. Dosing adjustments may be required in these instances. Displacement of bound drug does not inevitably lead to an increased free drug level, because free drug is subject to increased metabolism and elimination. Increases in plasma proteins and drug binding may also occur as an acute-phase response or during pregnancy, and, consequently, higher dosing may be necessary. Caution must be used when interpreting total drug levels in patients with possible protein disturbances or drug interactions, and free drug levels may be more useful in these situations.

Laboratory Methods

Currently, most clinical laboratories utilize immunoassays for the rapid and quantitative measurement of therapeutic drugs. In an immunoassay, drug in the patient's sample competes with a drug conjugate (drug attached to an enzyme or fluorescein molecule) for the binding of specific antibodies. Antibody binding results in blocking enzyme activity or in enhancing fluorescence polarization. By measuring enzyme activity or fluorescence polarization, the amount of drug in the patient sample is quantitated. Chemiluminescent immunoassays offering superior sensitivity are also available for drug analysis. Other immunoassay methods such as ELISA and radioimmunoassay are less commonly used. More complex laboratory techniques, such as chromatography with ultraviolet or mass spectral detection, are also commonly utilized for drug measurements. (See Chapter 2.) Immunoassays offer advantages over chromatographic methods, because immunoassays can be automated and analyze a greater number of samples more rapidly with less labor and cost. Only the total drug concentrations are routinely measured. Free drug levels require a more time-consuming and expensive ultracentrifugation or dialysis equilibrium steps to separate the protein-bound drug from the free drug. Free drug concentrations are typically lower than total drug concentrations by a factor of 2- to 20-fold, so more sensitive assays are required.

Specimen Collection

The appropriate specimen for therapeutic drug measurements is usually serum or plasma. Most laboratories do not accept gel separator tubes as the gel can bind drugs and interfere with drug recovery. Immunosuppressant levels are measured using whole blood due to the distribution and concentration of drug in RBCs, which are removed in the preparation of serum/plasma. EDTA-anticoagulated whole blood is the appropriate sample for these immunosuppressant drug measurements. Urine samples are frequently used to evaluate patient compliance in cases of therapeutic administration of buprenorphine, methadone, and several opiates (including oxycodone). Saliva or oral fluid may be appropriate for monitoring some medications, such as theophylline, in pediatric patients or in those for whom phlebotomy is difficult. Oral fluid is also not subject to adulteration or substitution, which can be an issue with monitoring for pain management compliance in patients prone to abuse. In general, trough levels are drawn just prior to the next dose and are used to evaluate the likelihood of a therapeutic effect. Peak levels are drawn at varying times, depending on the particular drug, and are used typically to assess toxicity risk.

Selected Commonly Monitored Drugs

Selected individual drugs and considerations for TDM are presented in Table 6–4. The required specimen volume and preservative will vary by analytical methodology, so the described collection instructions are only a guide. The reader should refer to specific instructions from the laboratory. The general monitoring recommendations will depend on the motivation for monitoring the drug, possible drug interactions, and whether the patient is stable or showing signs of toxicity. Therapeutic ranges are only suggestions and will vary by patient, condition, and the presence of other medications.

Methotrexate

Methotrexate is a folate antagonist used in the treatment of a wide variety of neoplasms. Dose-related toxicity is common with high-dose methotrexate therapy (defined as >1 g/m² or 20 mg/kg). Adverse effects include immunosuppression, and diverse organ damage including renal failure, myelosuppression, hepatic toxicity, neurotoxicity, gastrointestinal toxicity, and death. Toxicity correlates with serum methotrexate concentration and duration of exposure. Patients with poor hydration, renal insufficiency, pleural effusion, ascites, or gastrointestinal obstruction are at increased risk for toxicity. Adverse effects of methotrexate are ameliorated by administration of leucovorin, a reduced folate. Serial methotrexate levels are used to guide the appropriate dosing and duration of leucovorin rescue following high-dose methotrexate administration.

Immunosuppressants

The immunosuppressant drugs, tacrolimus (FK-506), cyclosporin, and sirolimus (rapamycin), are drugs used to prevent rejection in organ transplantation. Cyclosporin is also utilized to treat psoriasis, chronic autoimmune urticaria, and rheumatoid arthritis. These drugs were originally discovered in bacteria (tacrolimus and sirolimus) and fungus (cyclosporin) from soil samples. Monitoring is indicated because these drugs have a narrow therapeutic index and highly variable pharmacokinetics. Adverse effects include nephrotoxicity, hepatotoxicity, pulmonary toxicity, neurotoxicity (light sensitivity, tingling in the palms, and tinnitus), tremor, and hypertension.

Whole blood is the preferred specimen for TDM, as the immunosuppressant drugs concentrate into erythrocytes more than the plasma/serum portion of blood. Low trough concentrations may indicate subtherapeutic immunosuppression and can be associated with increased risk of rejection. High trough concentrations cause increased toxicity including nephrotoxicity that can be particularly challenging to diagnose in renal transplant patients. Drug levels must be interpreted in conjunction with other laboratory test results and clinical findings to discriminate between toxicity and rejection. For renal transplant patients on cyclosporin therapy, the only definitive method for differentiating graft rejection from drug-induced nephrotoxicity is renal biopsy. These drugs are sometimes used in combination, and with mycophenolic acid, to enhance the immunosuppressant effects and decrease the dose and side effects.

The immunosuppressants are extensively metabolized by the liver to a number of metabolites, some of which have immunosuppressant activity. Some metabolites can cross-react in laboratory immunoassays, thus overestimating parent drug concentrations in situations where elimination is impaired and when metabolites accumulate, as in cholestasis. Patients who have received mouse monoclonal antibody therapies may also have inaccurate immunoassay results. HPLC with tandem mass spectrometry is increasingly being used for laboratory analysis to circumvent cross-reactivity with the immunoassays.

Antibiotics

Aminoglycosides

Gentamicin, tobramycin, and amikacin are aminoglycoside antibiotics. Ototoxicity and nephrotoxicity from aminoglycosides are related to dose and duration of exposure. Numerous factors, such as renal and cardiac function, age, liver disease, and obesity, affect the pharmacokinetic properties of aminoglycosides. Because of the many patient factors, as well as the low margin of safety and high incidence of dose-related toxicity, aminoglycoside levels are usually indicated in conjunction with renal function monitoring to minimize toxicity. In patients with normal renal function and without underlying disease, the indication for drug monitoring is less well defined.

Vancomycin

Vancomycin is a tricyclic glycopeptide antibiotic with significant dose-related nephrotoxicity and ototoxicity. The practice of measuring vancomycin levels emerged from the guidelines for aminoglycoside monitoring. However, the necessity for vancomycin monitoring is controversial, because a good correlation between serum vancomycin levels and efficacy or toxicity has yet to be definitively demonstrated. Adult patients with normal renal function may not require routine monitoring. Indications for monitoring include impaired or changing renal function, concomitant use of nephrotoxic drugs, altered volume of distribution (as in a burn injury victim), prolonged vancomycin use, higher than usual doses, and use in neonates, children, pregnant women, and patients with malignancy.

Antiepileptics

Antiepileptics are frequently monitored to establish the dose necessary to reduce the frequency and magnitude of seizures. Trough levels are used to establish minimum effective dose. When toxicity is suspected, peak or random levels may be obtained. Too low a level will lead to breakthrough seizures, while too high a dose can induce seizures. A therapeutic level maintains seizure control and avoids side effects. The concentration of drug in the blood also may be used to evaluate patient compliance, and explain seizures that are refractory to drug treatment.

Phenytoin (Dilantin^®)

Phenytoin (or its prodrug phosphenytoin) is a widely used anticonvulsant with nonlinear kinetics and wide interindividual variability in dose requirement. Phenytoin toxicity includes ataxia, tremor, lethargy, seizure exacerbation, and neuropsychiatric changes. Phenytoin use in certain populations requires special consideration. Neonates and the elderly have decreased clearance. On the other hand, children metabolize phenytoin more rapidly than adults, and, therefore, dose adjustment is necessary at various ages. Careful monitoring in pregnancy is required due to metabolic and volume changes that occur during pregnancy. Phenytoin is highly protein bound, and conditions such as renal failure, liver disease, burn injury, and age will affect the amount of free drug by altering the amount of plasma protein.

Extensive protein binding also predisposes phenytoin to significant interactions with other protein-bound drugs, such as valproic acid. Coadministration of valproic acid and phenytoin is common and may cause a decrease in total phenytoin. Valproic acid displaces phenytoin from albumin, which causes a transient increase in free phenytoin, but this free phenytoin is readily metabolized and cleared. The overall effect is usually a decrease in total phenytoin with an unchanged level of free phenytoin. Monitoring of total phenytoin levels is sufficient for patient management, and free phenytoin levels are not usually necessary except in renal or hepatic disease, conditions that would affect total protein or body clearance.

Phenobarbital and Primidone (Mysoline®)

Phenobarbital and primidone are used to treat all types of seizures except absence (petit mal) seizures. The major active metabolite of primidone is phenobarbital. Clearance of both primidone and phenobarbital is prolonged in neonates, the elderly, and patients with hepatic and renal dysfunction. Phenobarbital is a potent hepatic enzyme inducer, and may affect the metabolism and levels of many other drugs metabolized by the same enzymes. Concurrent valproic acid use significantly decreases phenobarbital clearance.

Carbamazepine (Tegretol®)

The anticonvulsant carbamazepine is used not only for seizures but also for treatment of trigeminal neuralgia and bipolar disorder. Monitoring of carbamazepine levels is useful due to its slow and unpredictable absorption. Age and hepatic function affect drug clearance. Dose-related toxic effects include blurred vision, paresthesias, ataxia, nystagmus, and drowsiness. Carbamazepine is metabolized to the active metabolite, carbamazepine 10,11-epoxide. Children are known to accumulate the epoxide metabolite and, as a result, may present with toxicity in the setting of nonelevated carbamazepine levels. With chronic therapy, carbamazepine induces its own metabolism, and dosing adjustment becomes necessary.

Valproic Acid (Depakane®, Depakote®)

Valproic acid is used to treat all types of seizures. It is also used in the treatment of migraines and bipolar disorder. Valproic acid has a narrow therapeutic index. Dose-related adverse effects involve primarily central nervous system (CNS) depression. The average half-life of valproic acid is about 12 to 16 hours, but there is significant interindividual variability, and use of a sustained-release formulation is popular. The half-life of valproic acid is prolonged in neonates and in patients with liver dysfunction. Extensive protein binding accounts for the increased valproic acid toxicity observed in patients with uremia and cirrhosis.

Second-generation Antiepileptics

The second-generation antiepileptics encompass a range of drugs with different chemical structures and pharmacokinetics. Some are protein bound (lamotrigine is 55% bound to albumin) while others are not (levetiracetam is <10% protein bound). Common adverse effects include dizziness, ataxia, nausea, and vomiting. Decreased hematocrit and neutropenia can also be seen with lamotrigine. In general, the second-generation antiepileptics have a wider therapeutic index and fewer side effects than the first-generation drugs. HPLC and immunoassays are available. However, therapeutic and toxic ranges have not been established for all of these drugs. So, monitoring is generally conducted to define the individual level at which a patient is achieving therapeutic action with fewest side effects for future reference, for compliance, and for documentation of the level at which side effects are evident for that patient.

Antidepressants

Tricyclic Antidepressants

Tricyclic antidepressants are monitored for multiple reasons. There is significant interindividual variation in metabolism and elimination, such that standard dosing results in therapeutic levels in less than half of patients. Genetic variation accounts for some of this variability. The fraction of “poor metabolizers” is about 17% of Caucasians and 5% of other ethnic groups. Other indications for monitoring include a narrow therapeutic index, multiple drug interactions, and patient compliance.

Tricyclic antidepressants have a low margin of safety and cause anticholinergic toxicity, seizures, and arrhythmias in overdose. Although the correlation between toxicity and blood level is poor, there are general guidelines. Levels in excess of 500 μg/L may be associated with anticholinergic toxicity (flushing, tachycardia, fever, dilated pupils, dry mucous membranes, urinary retention, and absent bowel sounds). Cardiotoxicity is more likely to occur at levels greater than 1000 μg/L in acute overdose.

Lithium

Lithium is a univalent cation most commonly used to treat bipolar disorder. Lithium monitoring is useful due to its narrow therapeutic index and the wide interindividual variation in dose requirement.

Excretion of lithium is primarily renal. Children have increased clearance, while the elderly have decreased clearance. Lithium excretion parallels sodium excretion. Therefore, patients on stable doses of lithium may become toxic in states of sodium conservation such as fever, excessive sweating, lack of fluid intake, and diarrhea.

Toxicity is usually associated with levels in excess of 1.5 mmol/L. However, toxicity may occur at lower levels, especially in cases of chronic toxicity. Lithium overdose is characterized by lethargy, weakness, slurred speech, ataxia, tremor, and myoclonic jerks. Severe toxicity may result in seizure, hyperthermia, and coma. Management of patients who have ingested sustained-release lithium preparations is difficult, and serum measurements play a crucial role in the decision to instigate hemodialysis or whole bowel irrigation. Analytical methods involve the use of ion-specific electrodes, and spectrophotometry or colorimetric tests.

Later-generation Antidepressants

Fluoxetine was the first selective serotonin-reuptake inhibitor used to treat depression. Fluoxetine monitoring is useful when patient compliance is in question. Further monitoring is not likely to be beneficial since fluoxetine has a wide therapeutic index, and there is a poor correlation between blood levels and clinical response. Fluoxetine is metabolized by the liver to the active metabolite norfluoxetine.

Other serotonin-reuptake inhibitors/later-generation antidepressants—such as sertraline (Zoloft®), paroxetine (Paxil®), fluvoxamine (Luvox®), citalopram (Celexa®), quetiapine (Seroquel®), trazodone (Deseryl®), and venlafaxine (Effexor®)—do not require routine monitoring due to their wide therapeutic indices.

Cardiac Drugs

Digoxin

Digoxin is a commonly used drug in the treatment of heart failure and arrhythmias, and it has a low therapeutic index. There is significant interindividual variation in digoxin absorption and distribution along with prolonged clearance in patients with impaired renal function. Digoxin overdose is characterized by gastrointestinal distress, confusion, visual changes, hyperkalemia, and life-threatening cardiac toxicity. Overdoses may be treated with an antidigoxin antibody antidote. Such treatment typically renders subsequent blood digoxin concentrations unreliable. Blood digoxin immunoassays are generally less reliable than immunoassays for other therapeutic agents. Interferences with digoxin immunoassays are frequently reported. These interferences are referred to as digoxin-like immunoreactive substances (“DLIS”).

Pain Management

Acetaminophen

Acetaminophen is a therapeutic drug used as an analgesic and an antipyretic. When it is used in the recommended doses, it is not necessary to measure acetaminophen levels. However, excess intake of acetaminophen can be associated with severe liver injury. Thus, acetaminophen is a representative of many compounds with a wide therapeutic window that does not require therapeutic monitoring when used in recommended doses. However, because toxicity can occur if the upper limit of the window is exceeded, monitoring acetaminophen levels in patients with excess intake is critical, particularly since an antidote to the major toxic effect can be administered. Table 6–5 presents an overview of the laboratory evaluation for acetaminophen toxicity. Immunoassays are available for the rapid determination of serum/plasma levels.

Acetaminophen is rapidly absorbed from the gastrointestinal tract. The plasma concentration reaches its highest level 30 to 60 minutes after a dose. One of the compounds resulting from acetaminophen metabolism is an oxidation product that is hepatotoxic. Normally this metabolite is detoxified by binding to glutathione in the liver. With excess intake of acetaminophen, the production of the toxic metabolite exceeds the amount of hepatic glutathione, and this permits the toxic metabolite to produce liver injury. Renal damage also may occur as a result of injury by the same compound.

The recommended daily dose of acetaminophen is no more than 4 g per day. A single dose of 10 to 15 g may produce liver injury. Fatal disease is usually associated with ingestion of ≥25 g of acetaminophen. Acetaminophen at slightly more than the recommended 4 g per day can produce hepatotoxicity when the patient has also ingested ethanol, and this response can be exacerbated if the patient had been fasting prior to ingestion of acetaminophen and ethanol, or takes another enzyme-inducing drug such as phenytoin. The ingestion of acetaminophen at greater than recommended doses produces corresponding elevations of acetaminophen in the blood, and the level of the drug in the blood correlates with the severity of hepatic injury.

Acute manifestation of excess acetaminophen intake typically occurs 2 to 3 hours after ingestion. Most often this includes nausea, vomiting, and abdominal pain. Cyanosis of the skin and fingernails may be observed as a result of methemoglobin generation from the overdose. The full extent of liver damage usually becomes apparent 2 to 4 days after drug ingestion. At that time, liver function test results, including the prothrombin time, become abnormal. A variety of associated abnormalities, including electrolyte disturbances, can occur if there is significant liver damage. Acute renal failure also may occur, even if liver failure is not observed.

Aspirin

Aspirin (acetylsalicylic acid) is a therapeutic drug in use for more than a century as an analgesic, antipyretic, anti-inflammatory, and antithrombotic agent. It is readily absorbed and rapidly metabolized by hydrolysis to salicylic acid. Peak concentrations occur within 1 to 2 hours with a therapeutic dose. Between 50% and 90% is bound to albumin in a dose-dependent manner. Further metabolism produces salicyluric and gentisic acids and glucuronide conjugates that are renally excreted. Aspirin is contained in many preparations, including those with other analgesics. When used in therapeutic doses, it is not necessary to measure levels. However, chronic salicylate poisoning (salicylism) carries a high morbidity (30%) and mortality (25%), and is difficult to diagnose without monitoring levels since the patient may be too confused to give a reliable history. Table 6–6 presents an overview of the laboratory evaluation for salicylate toxicity. Immunoassays are available for the rapid determination of serum/plasma levels. About 500 mg/kg as an acute dose is potentially lethal in comparison to a normal dose of 15 mg/kg. When taken in therapeutic doses, the half-life is 2 to 5 hours, but metabolism becomes saturated once the dose exceeds about 30 mg/kg, causing a delay in drug elimination. An early feature of toxicity is respiratory alkalosis through direct stimulation of the respiratory drive center, followed by vomiting. The latter mechanism of toxicity results from uncoupling of oxidative phosphorylation, leading to ketosis, metabolic acidosis, and pyrexia, with further dehydration and electrolyte imbalance. Hematologic consequences arise that manifest as an increased prothrombin time, GI bleeding, and occasionally DIC.

Other Pain Management Drugs

Buprenorphine and methadone are analgesics that are commonly utilized for opiate withdrawal, but have found recent medical application in the management of chronic pain. Fentanyl and oxycodone are other drugs utilized in pain management that may be monitored. Serum/plasma levels of these drugs correlate poorly with clinical effect because of tolerance. The safety of these drugs in doses utilized for pain management does not typically require monitoring and dosing can be adjusted based on pain relief. However, these drugs have high abuse potential, so urine tests are sometimes used to monitor for compliance and ensure that the patient is not diverting the drug for sale or other purposes. Immunoassays are available for analysis of these drugs in urine samples.

Overview

Drug of abuse testing (DAT) includes testing for the use of illicit drugs (eg, cocaine and phencyclidine [PCP]) and potentially addictive or harmful therapeutic agents (eg, benzodiazepines, opiates, and amphetamine). The goal of DAT is to detect past exposure or use of a drug. Quantitative levels of a drug or its metabolite in fluids are not required. Laboratory analysis determines if a drug is above (ie, “present”) or below (absent) a defined cutoff concentration in the sample.

DAT assays are designed to detect either the parent drug or metabolite with a longer half-life. Cocaine, for example, has a blood half-life of less than 60 minutes, but the principal metabolite, benzoylecgonine, has an 8-hour half-life. By detecting the metabolite, cocaine abuse can be detected for several days after use as opposed to the parent drug that is cleared in less than 5 hours. Table 6–7 lists the typical detection window for analysis of common drugs of abuse.

Drugs and their metabolites are much more concentrated in urine than in serum. Urine is the specimen of choice for DAT testing because of its ready availability. Other specimens have also been utilized for DAT. Meconium (an infant's first bowel movement after birth) has been utilized to detect in utero exposure of a fetus to drugs. Since meconium is made during the last trimester of fetal development, drug exposure can be detected from as early as the sixth month of gestation. Hair and nails have also been analyzed in attempts to detect drug use over a longer time frame than urine, but the clinical utility of specimens other than urine remains controversial.

Analysis for multiple drugs in a patient's sample can be labor intensive and expensive. Laboratory analysis for drugs of abuse thus takes a 2-tier approach for efficiency. A simple, rapid assay that can be readily automated is used to first screen a sample for the presence of a class of drugs. These screening tests are sensitive and designed to detect a broad range of similar drugs that share a common chemical structure. Unfortunately, screening tests are generally not very specific and subject to a variety of cross-reactivities that can lead to false-positive test results. Thus, a second tier of more specific testing is conducted to “confirm” the presence of a particular drug in samples that screen positive. Common screening tests are immunoassays that can be readily performed on chemistry instrumentation in a clinical laboratory or on point-of-care devices for testing in a variety of settings outside of a formal laboratory. Table 6–8 lists the cutoffs and other characteristics of immunoassay tests. Most immunoassays detect a number of drugs within a class of agents, but some immunoassays are specific to a given compound, like the metabolite of cocaine, PCP, oxycodone, and 6-monoacetylmorphine.

The second-tier or “confirmatory” testing employs a chromatographic separation prior to mass-spectrometric detection to exactly identify the drug or metabolite in samples that generated a positive immunoassay screen. (See Chapter 2.) Confirmatory testing is expensive and time-consuming, but is absolutely required for forensic, legal, pre-employment, and other applications of the test result that mandate definitive analysis. Confirmatory testing can take several days and will not assist in the immediate care and management of a trauma or acute overdose patient.

The lack of confirmatory testing on clinical samples can lead to misinterpretation of test results unless the clinician maintains an active dialogue with the laboratory regarding the likely causes of false-positive screening tests. Many laboratories append a comment to warn that positive urine immunoassay results have not been confirmed. Common causes of false-positive test results may also be listed with the result.

Common drugs that cause cross-reactivity and false-positive DAT results are listed in Table 6–8. Cross-reactivity and the causes of false-positives are dependent on the antibody specificity employed in the immunoassay, so cross-reactivities will vary from 1 manufacturer to another and between laboratories based on the method utilized for analysis. Clinicians should be familiar with the characteristics and limitations of the drug tests employed to analyze patient specimens.

Drugs of abuse have no reference or therapeutic range, because the drug should not be detected in the patient's sample. However, sometimes a positive drug test is desired by the ordering physician. Urine oxycodone, for example, is utilized for pain management, but can also be sold by a patient for monetary profit. A positive urine oxycodone test is targeted to assure that patients are compliant with their drug regimen and not diverting the drug for other purposes.

Screening for drugs of abuse does not detect every possible drug that the patient may have ingested. Laboratory analysis is limited to the specificity of the tests at hand. Patients may have access to a wide variety of plants that contain toxins or medications for which the laboratory does not test. In general, drug analysis is designed for rapid identification that may permit effective treatment (eg, acetaminophen and its antidote) to reduce the toxic effects of the compound. Concomitant ingestion of different drugs is very common and may play a role in laboratory analysis for drugs of abuse. For example, ethanol and cocaine are often ingested together, and patients ingesting these drugs together can form a toxic metabolite called cocaethylene.

Specimen Collection and Laboratory Analysis

The potential for sample adulteration is a concern for any DAT. False-negative results can be caused by purposely “adulterating” a urine sample to prevent the immunologic or indicator reaction from working, leading to a false-negative result despite drug in the sample. Common household chemicals such as bleach, vinegar, sodium bicarbonate, Drano, soft drinks, or hydrogen peroxide may be added to a urine sample in an effort to cause false-negative results. Many of these additives work by changing the sample pH, to denature the antibody proteins used in the assay or to shift the pH away from optimum assay conditions. Such adulteration can be detected by checking the pH of urine samples submitted for drug testing. Other adulterants include glutaraldehyde or nitrites, and can be analyzed using specific tests for adulteration in the laboratory.

Patients can take diuretics to temporarily enhance elimination. Diuretics cause an increase in urine output, so drugs that may be present will be diluted below the assay cutoff concentration in the urine sample. Alternatively, patients may submit water as their urine sample, so as to avoid detection of drug use. Collection facilities can deter sample dilution by monitoring the temperature of samples just after collection. Samples outside a physiologic temperature range should be suspected of dilution. Facilities can also cap hot water faucets and add bluing agents to the toilet water as additional deterrents. Laboratories can test for urine osmolality and look for specimen dilution by analysis of urine creatinine.

Patients may also try to avoid detection by substituting specimens. Submission of someone else's urine as a patient sample is perhaps the hardest for laboratories to detect. Sources of drug-free urine are readily available via the Internet, and these materials are used by patients trying to avoid detection of their drug use. In nearly all standard tests, these materials act like normal, unadulterated human urine. Without close monitoring of a patient during urine sample collection, sample substitution is nearly undetectable by laboratory methods.

When evaluating a patient for drug use, collecting an appropriate specimen is an important step to detecting the presence of a drug.

• Most drugs and metabolites are concentrated in urine after use. Urine is appropriate for qualitative analysis (presence/absence of drug) and for determining recent use of amphetamines, benzodiazepines, barbiturates, cannabinoids, cocaine metabolites, opiates, and their metabolites (including codeine and morphine), oxycodone, and PCP. Urine is easy to collect and noninvasive, but subject to adulteration, dilution, and sample substitution. Monitoring of urine collection is intrusive of patient privacy, so facilities collecting urine samples should take steps to deter adulteration (capping hot water faucets and using bluing agents in the toilets) and have patients remove coats and bulky clothing, and keep purses and backpacks out of the bathroom during collection.

• Serum/plasma and blood samples provide a single time point of drug in the patient's system. Since blood is in equilibrium with tissue and organ receptors at steady state, quantitative blood levels of drug can assess for intoxication and toxicity. Serum, plasma, and blood concentration are useful for analyzing alcohol intoxication, management of analgesic overdose (including acetaminophen and salicylates), and evaluation of TDM side effects and toxicity. Collection of blood requires a needlestick and phlebotomy, but is not subject to sample adulteration like urine samples.

• Other body fluids may prove useful: meconium for evaluation of in utero exposure to drugs, vitreous humor for postmortem examination, hair and nails for past exposure to certain drugs of abuse and toxins, and sweat or oral fluid that can be collected without invasion of privacy and are less prone to adulteration.

It is essential that specimens are collected at an appropriate time following ingestion and that the sample is properly preserved. Serial measurements over time may be necessary because of delayed gastric emptying or prolonged absorption from sustained-release preparations in overdose cases. Blood levels do not necessarily correlate with the severity of toxicity because many compounds distribute into specific body compartments or cells, and are therefore less detectable in blood. The detection of drugs in urine will depend on the patient's renal function and urine output, the time since last dose, chronic use of the drug, the patient's hydration state, and metabolism. Thus, urine collection within the first several hours after use has a better chance of detecting drug than urine collected several days after use. Not all drugs are equally stable in a body fluid. Alcohols should be collected anaerobically, as they are volatile compounds that will dissipate when exposed to air. Appropriate steps should be taken to preserve the drug in the specimen after collection prior to analysis.

Selected Drugs of Abuse

Amphetamines

Amphetamines are stimulants and common class of abused drugs. Methamphetamine (crank, speed), 3,4-methylenedioxymethamphetamine (a derivative of methamphetamine, also known as MDMA or ecstasy), and several other amphetamine derivatives are used orally and intravenously as illicit drugs. A smokable form of methamphetamine is known as “ice.” Amphetamine-like drugs also can be used as prescription medications for treatment of a variety of conditions and disorders. These include weight loss, narcolepsy, attention deficit disorders, and sinus congestion. Amphetamine-like drugs work primarily by activating the sympathetic nervous system via the CNS. Drugs in this class can produce toxicity at levels only slightly above the usual doses, but a high degree of tolerance can develop after repeated use. Patients who are intoxicated with amphetamine-like drugs present with CNS effects that can extend from euphoria to seizure and coma. More severe signs and symptoms are usually associated with greater amounts of drug ingestion. The acute peripheral manifestations extend from sweating and tremor to myocardial infarction, even if the coronary arteries are normal. Death in amphetamine users can be caused by ventricular arrhythmia. The ingestion of amphetamines and related drugs can be conclusively established by identification of these compounds in urine or in gastric samples. Quantitative serum levels often do not correlate with the severity of the signs and symptoms.

Barbiturates

Barbiturates are used clinically as hypnotic and sedative agents, for induction of anesthesia, and for treatment of epilepsy. Ultrashort-, short-, intermediate-, and long-acting barbiturates have different pharmacokinetic properties. All barbiturates cause a generalized depression of neuronal activity in the brain. The toxic dose of barbiturates depends on the specific barbiturate used, the route and rate of administration, and individual patient tolerance. Toxicity is likely to appear when the dose administered exceeds 5 to 10 times the hypnotic dose, but chronic use may result in marked tolerance. The patient with mild-to-moderate intoxication of barbiturates often presents with lethargy, slurred speech, nystagmus, and ataxia. With greater amounts of drug ingestion in overdose, hypotension, coma, and even respiratory arrest can occur. Barbiturates can be detected in both urine and serum to document their ingestion.

Benzodiazepines

Benzodiazepines are sedatives, and antiepileptic and antianxiety medications. The different benzodiazepines vary in potency, duration of effect, and conversion to active and inactive metabolites. Benzodiazepines produce a generalized depression of spinal reflexes and may cause coma. Death from benzodiazepine overdose is rare unless the drugs are used in combination with other compounds, such as alcohol. Oral overdoses of diazepam (Valium®) have been reported in excess of 15 to 20 times the therapeutic dose without serious depression of consciousness. However, if the same drug is given at a much lower concentration with rapid intravenous injection, respiratory arrest can occur. Although there is variability among benzodiazepines, the onset of CNS depression is typically observed 30 to 120 minutes after ingestion. Drug levels can be obtained from both serum and urine specimens. However, because levels are rarely of value in emergency management, quantitative analysis is often not conducted. Of the benzodiazepines, clonazepam, used in the treatment of absence seizures, is the most often monitored, especially in children, although most patients are managed without monitoring. There is little evidence for a therapeutic window, probably because receptor effects do not mirror plasma concentrations, and tolerance can develop with continued use. HPLC is used for quantitative analysis in serum/plasma, but the presence of drug can be detected in urine by immunoassay.

Cannabinoids

Cannabis derivatives include marijuana and hashish. Marijuana consists of the leaves and flowering parts of the plant Cannabis sativa. Marijuana is usually smoked in cigarettes or pipes, and can be ingested in food. Dried resin from the plant can be compressed into blocks to make hashish. The primary psychoactive cannabinoid in marijuana is delta-9-tetrahydrocannabinol (THC). THC is also available in capsule form as a treatment for nausea in patients being treated with chemotherapeutic agents and those undergoing treatment for glaucoma. The effects of THC are related to the dose and time after consumption. THC may be a stimulant, a sedative, or a hallucinogenic compound. A typical marijuana cigarette contains 1% to 3% THC, but some may contain up to 15% THC. Hashish contains 3% to 6% THC, and an oil extract can be prepared from hashish with 30% to 50% THC. Significant variability in toxicity exists between individuals, which is influenced by prior exposure to the drug and degree of tolerance. The clinical presentation of a patient after use of THC can vary from euphoria and a heightened sensory awareness to impaired short-term memory, depersonalization, visual hallucinations, and acute paranoid psychosis. THC use can be established by detection of the drug in the urine. However, drug levels correlate poorly with the degree of intoxication. A urine test for cannabinoids may be positive for 10 to 25 days after the last exposure in moderate to heavy use. In fact, there are well-documented reports of true-positive test results in chronic use more than 80 days after last exposure to THC.

Cocaine

Cocaine is a stimulant drug. It may be sniffed into the nose, smoked, or injected intravenously. The “free base” form of cocaine is preferred for smoking because it volatilizes at a lower temperature and is not as easily destroyed by heat as the hydrochloride salt of the drug. Crack cocaine is a dried form of the drug that has been mixed in alkaline aqueous solution to generate the free base. Cocaine can also be combined with heroin and injected as a “speed ball.” The primary effect of cocaine is generalized sympathetic stimulation, very similar to that produced by amphetamines. There is also a depression of cardiovascular function as a result of decreased cardiac contractility. The toxic dose depends significantly on the tolerance of the individual to the drug, the route of administration, and whether the cocaine is administered with other compounds. A dose that produces only euphoria when swallowed or snorted can produce convulsions and cardiac arrhythmias when rapidly injected intravenously or smoked. The initial euphoria from exposure to cocaine can be followed by anxiety, agitation, hyperactivity, and seizures. With high doses, respiratory arrest can occur. Death can result from fatal arrhythmia, status epilepticus, intracranial hemorrhage, or hyperthermia. Cocaine use can be detected through analysis of the primary metabolite, benzoylecgonine, in the urine, although parent drug and metabolites can also be analyzed in plasma/serum or in vitreous humor in death investigations.

Opiates and Opioids

Opiates are narcotic sedatives and analgesics used for pain management. They are naturally occurring compounds extracted from the poppy Papaver somniferum. Opioids include the naturally occurring opiates and their derivatives (morphine and codeine) and the synthetic opioids (dihydrocodeine, heroin, hydrocodone, hydromorphone, oxycodone, and oxymorphone). Many prescription medications contain opioids. Mixtures of aspirin or acetaminophen with an opioid compound, such as codeine, are in common use. Dextromethorphan is an opioid derivative that is used to suppress cough. This compound can be obtained without a prescription as it has no analgesic or addictive properties. Morphine is an opiate that is widely used in medicine to reduce pain. The best known drug of abuse in this category is heroin. In general, all opiates and opioids cause sedation and respiratory depression. Toxicity is related to respiratory failure that can lead to death. The toxic dose varies widely with the opioid administered, its route and rate of administration, and the individual's tolerance to the drug. Diagnosis of opiate intoxication may be established clinically when the typical signs and symptoms are present—pinpoint pupils, and respiratory and CNS depression. These symptoms are reversed by administration of the opioid antagonist naloxone. Opioid use can be determined in urine using immunoassay. Specific immunoassays can detect the presence of methadone, fentanyl, and buprenorphine while chromatographic techniques can analyze meperidine and tramadol. Levels of these compounds in serum/plasma, however, are not usually analyzed, because opiate concentration correlates poorly with clinical effects.

Oxycodone is synthesized from thebane, a natural constituent of opium. It is available in a number of compound analgesics, with acetaminophen and aspirin. Oxycodone abuse has grown over the last decade, and specific immunoassays are available for this drug, since most broad-spectrum opiate class immunoassays fail to detect oxycodone in the clinically relevant concentration range.

Phencyclidine

PCP is an anesthetic agent that became popular as an inexpensive street drug in the late 1960s. It is most often smoked, but it can be snorted, ingested orally, or injected. It is commonly used in combination with other illicit drugs. Ingestion of PCP produces a generalized loss of pain perception and can cause hallucinations, euphoria, and disinhibition. Ingestion of large amounts can produce death, often from self-destructive behavior or from complications of hyperthermia. PCP use can be detected in the urine using immunoassays. PCP levels in the serum are not clinically valuable, because drug levels do not correlate with the degree of intoxication.

Alcohols: Ethanol, Methanol, Ethylene Glycol, and Isopropanol

Ethanol is the most common drug of abuse. Many patients presenting to hospitals with altered mental status suffer from excess ethanol ingestion. Ethanol is present not only in beverages but also in many medications. Ethanol intoxication is associated with many different types of accidental injury, particularly those involving motor vehicles. Chronic abuse of ethanol can lead to pancreatic disease and liver cirrhosis.

Ethanol is rapidly absorbed from the gastrointestinal tract. It distributes into the total body water and diffuses freely into the tissues. Peak blood ethanol levels occur 30 to 75 minutes after ethanol ingestion. Food ingestion can delay absorption. A useful rule of thumb is that 1 oz of 80 to 100 proof spirits, 4 oz of wine, or 12 oz of beer increases the blood alcohol concentration by 25 to 30 mg/dL when ingested over a period of several minutes. The blood ethanol level in a nonchronic alcoholic decreases at a rate of 15 to 25 mg/dL/h once ethanol ingestion is discontinued. The blood ethanol levels required to induce fetal alcohol syndrome have not been determined. Pregnant women who abuse ethanol have a high risk of delivering an infant with fetal alcohol syndrome. These infants have prenatal growth retardation, dysfunction of the CNS, and characteristic craniofacial abnormalities. Because an acceptable lower limit of alcohol intake in pregnancy has not been defined, pregnant women are generally advised to abstain from ethanol.

Metabolism of ethanol occurs by an oxidative pathway to acetaldehyde and acetic acid, and, also, by a nonoxidative pathway to fatty acid ethyl esters. When the acetaldehyde is subsequently converted to acetic acid, acidosis can occur. The major metabolizing enzyme for oxidation of ethanol to acetaldehyde is alcohol dehydrogenase, and a second enzyme is the cytochrome P450 system. The cytochrome P450 enzymes are liver microsomal enzymes involved in the metabolism of several drugs. Cytochrome P450 enzymes can be induced to higher levels of activity by ethanol. Ingestion of ethanol can thus alter the metabolism of a number of drugs, which are metabolized by this same system. Induction may increase or decrease the therapeutic and/or toxic effect of drugs. Ethanol can also compete with drugs for metabolism by the cytochrome P450 enzymes.

The measurement of blood ethanol concentration can be performed by breath analysis, or more accurately, by a specific enzymatic assay or by gas chromatography that can also measure ethanol, methanol, and isopropanol, individually (Table 6–9). Methanol and isopropanol are also toxic, primarily as a result of oxidative metabolism. Methanol metabolism can generate formic acid leading to blindness, while isopropanol is metabolized to acetone causing ketosis. Ethylene glycol, another volatile compound, is oxidized by similar metabolism to oxalic acid causing acute kidney failure. Gas chromatography under different laboratory conditions is required to detect ethylene glycol. The clinical presentation, sources for ingestion, and laboratory detection and quantitation of methanol, ethylene glycol, and isopropanol are shown in Table 6–10.

Overview

Monitoring of environmental toxins is a significant challenge because so many substances that can produce illness and even death are encountered in daily life. Occupational exposure to heavy metals, gases, and caustic compounds can occur in the workplace, and leaching of toxins from an industry can contaminate soil and groundwater, leading to exposure of food sources, drinking water supplies, and livestock. Carbon monoxide, mercury, cyanide, and insecticides are some of the notable environmental toxins. Lead exposure can occur from occupational and nonoccupational sources (such as paint chips) and produce subclinical to life-threatening illness, depending on the amount ingested. Low-level exposure in children can produce serious disease and affect long-term mental development.

Laboratory analysis of an environmental toxin may not always measure the compound directly. In some cases, the toxin impairs flow through a metabolic pathway. Accumulated metabolites can be measured that reflect the toxic effects rather than a direct analysis of the toxin. Insecticide exposure, for instance, can be measured indirectly by cholinesterase enzyme activity rather than direct analysis of the insecticide levels in the body.

Carbon Monoxide

Description

Carbon monoxide poisoning is responsible for up to 4000 deaths per year in the United States and is the leading cause of accidental and deliberate poisonings. Approximately 10% of the cases involve children. The heart, CNS, and lungs are the organs most immediately affected by the toxic effects of carbon monoxide. Carbon monoxide can also impair vision, hearing, and peripheral nerve conduction. The poisoning may be sublethal and cause cardiac dysrhythmias, myocardial ischemia, headache, and a variety of other signs and symptoms. Survivors can suffer permanent, severe neurologic impairment.

The principal pathologic consequence of carbon monoxide poisoning is the binding of carbon monoxide to oxygen-binding sites in the hemoglobin molecule. The binding of carbon monoxide to hemoglobin results in the formation of carboxyhemoglobin. Carbon monoxide has a higher affinity for hemoglobin than oxygen and decreases the hemoglobin's ability to deliver oxygen to the tissues producing ischemia.

The normal range for percent carboxyhemoglobin for adults is 0.1% to 0.9% of total hemoglobin. When hemolytic disease is present with increased breakdown of hemoglobin, the carboxyhemoglobin levels can increase to approximately 2%. Values at this level can have adverse clinical effects in patients with preexisting heart disease. There is a poor correlation between carboxyhemoglobin levels and clinical findings. Table 6–11 shows the relative consequences of various amounts of carboxyhemoglobin, but individual patients demonstrate considerable variability in clinical symptoms. Children are much more susceptible to acute carbon monoxide poisoning and have a different clinical picture that mimics gastroenteritis. Like adults, they can also have serious neurologic sequelae and myocardial ischemia. Patients are often unaware of their exposure to carbon monoxide because it is odorless and nonirritating. There is no pathognomonic feature of carbon monoxide intoxication. A rapid diagnosis of carbon monoxide poisoning is important in order to institute appropriate management and identify sources of carbon monoxide before other exposures can occur.

Diagnosis

Laboratory monitoring of carbon monoxide poisoning is performed by measurement of the carboxyhemoglobin levels (Table 6–12). The patient also must be evaluated for possible underlying cause(s) of an increased carbon monoxide level (<10%), such as the presence of anemia, infection, and smoking that can increase carboxyhemoglobin levels. Because carbon monoxide can cause ischemic damage to skeletal and cardiac muscle, evaluation of ischemic muscle damage may also be appropriate.

Lead

Description

Lead poisoning is primarily a disease of childhood. As research has increased about lead toxicity, the threshold for defining lead poisoning has decreased over the past 20 years. Prior to 1970, lead poisoning (plumbism) was defined by blood levels greater than 60 μg/dL. In 1971, the threshold was lowered to 40 μg/dL. By 1975, the acceptable level was 25 μg/dL and since 1985, blood levels of 10 to 15 μg/dL have been recognized to impair cognitive and behavioral development. Most recently, the Centers for Disease Control and Prevention have adopted recommended lead levels in children of <5 μg/dL. As recently as 1992, 17.2% of children in the United States between the ages of 6 months and 5 years were estimated to have a blood lead level in excess of 15 μg/dL, although this pales in comparison to those observed in developing countries. These children were primarily from low-income families in large urban settings. The sources of lead for these children included not only lead paint but also lead-contaminated household dust, soil, and workplace clothing; the use of lead-containing cookware; exposure to lead in storage batteries, in fishing and curtain weights; and even lead-contaminated water from the use of lead-soldered pipes in older buildings. Some canned food has been reported to contain lead. Lead-containing costume jewelry, medicines, and cosmetics (such as surma or kohl) imported to the United States from other countries have been demonstrated to contain lead. Obviously, the investigation of lead poisoning cases is important to identify the precise source of lead ingestion so that exposure can be eliminated.

There is a significant effort nationally to screen children, particularly those between the ages of 6 months and 5 years who live in, or are frequent visitors to, deteriorated housing built prior to 1960. Exposure to lead at high doses can produce persistent seizures, mental retardation, and chronic behavioral dysfunction. Most of the absorbed lead is stored in the bones. However, lead can also be found in soft tissues and erythrocytes. Lead interferes with the enzymes involved in heme synthesis, so lead exposure can lead to anemia. Renal toxicity is also observed in some cases of chronic lead poisoning. Any child with developmental delay, behavioral disorders, seizures, learning disabilities, iron deficiency, hearing impairment, renal disorders, and recurrent vomiting and abdominal pain should be considered for lead toxicity.

Diagnosis

A whole blood lead level reflects the lead burden of the body. For small children, a finger-stick sample is usually sent for analysis. Table 6–13 describes the laboratory monitoring for children suspected of lead poisoning relative to the presenting blood lead level. If the blood lead level is greater than 5 μg/dL, testing of a sample taken by venipuncture is recommended to rule out skin contamination. If this “clean” specimen contains more than 5 μg/dL lead, parental education on possible exposure sources is recommended. While initial testing is often performed by anodic stripping voltammetry, confirmation testing especially for concentrations above 40 μg/dL is generally performed by atomic absorption or mass spectrometry. Free erythrocyte protoporphyrin (zinc protoporphyrin) is formed from heme synthesis as a result of lead toxicity. The measurement of free erythrocyte protoporphyrin is, however, an insensitive screening test for lead exposure because it does not detect lead poisoning in children with lead levels between 10 and 25 μg/dL, and identifies less than 50% of children with blood levels greater than 25 μg/dL. The utility of free protoporphyrin measurement is primarily to detect ongoing lead exposure. Because the anemia from lead poisoning may resemble iron deficiency anemia, studies for iron deficiency (such as serum ferritin and the red cell distribution width in the complete blood count) should be obtained to differentiate anemia of lead poisoning from iron deficiency anemia.

A portion of this chapter on TDM is also found in a newsletter to the physicians at the Massachusetts General Hospital in Clinical Laboratory Reviews, 1999;8:1.