Salicylate, salicylamide, methyl salicylate (each will increase acetaminophen level by 10% of their level in mg/L); bilirubin; phenols; renal failure (each 1-mg/dL increase in creatinine can increase acetaminophen level by 30 mg/L).
Other volatile stimulant amines (misidentified). GC mass spectrometry poorly distinguishes d-methamphetamine from l-methamphetamine (found in Vicks inhaler).
All assays are reactive to methamphetamine and amphetamine, as well as drugs that are metabolized to amphetamines (benzphetamine, clobenzorex, famprofazone, fenproporex, selegiline). The polyclonal assay is sensitive to cross-reacting sympathomimetic amines (ephedrine, fenfluramine, isometheptene, MDA, MDMA, phentermine, phenmetrazine, phenylpropanolamine, pseudoephedrine, and other amphetamine analogs); cross-reacting nonstimulant drugs (bupropion, chlorpromazine, labetalol, ranitidine, sertraline, trazodone, trimethobenzamide). The monoclonal assay is reactive to d-amphetamine and d-methamphetamine; in addition, many have some reactivity towards MDA and MDMA.
Oxaprozin. Note that many benzodiazepine assays give false-negative results for drugs that do not metabolize to oxazepam or nordiazepam (eg, lorazepam, alprazolam, others).
Bromide (variable interference).
Ketoacidosis (may increase creatinine up to 2–3 mg/dL in non-rate methods); isopropyl alcohol (acetone); nitromethane (up to 100-fold increase in measured creatinine with use of Jaffe reaction); cephalosporins; creatine (eg, with rhabdomyolysis).
Creatine, lidocaine metabolite, 5-fluorouracil, nitromethane “fuel”
Endogenous digoxin-like natriuretic substances in newborns and in patients with hypervolemic states (cirrhosis, heart failure, uremia, pregnancy) and renal failure (up to 0.5 ng/mL); plant or animal glycosides (bufotoxins; Chan Su; oleander); after digoxin antibody (Fab) administration (with tests that measure total serum digoxin); presence of heterophile or human antimouse antibodies (up to 45.6 ng/mL reported in one case).
Falsely lowered serum digoxin concentrations during therapy with spironolactone, canrenone.
Other alcohols, ketones (by oxidation methods).
Isopropyl alcohol; patients with elevated lactate and LDH.
Other glycols, elevated triglycerides.
Propylene glycol (may also decrease the ethylene glycol level).
Glucose level may fall by up to 30 mg/dL/h when transport to laboratory is delayed. (This does not occur if specimen is collected in gray-top tube.)
Deferoxamine causes 15% lowering of total iron-binding capacity (TIBC). Lavender-top Vacutainer tube contains EDTA, which lowers total iron.
Skin disinfectant containing isopropyl alcohol used before venipuncture (highly variable, usually trivial, but up to 40 mg/dL).
Acetylcysteine, valproic acid, captopril, levodopa. Note: Acetest method is primarily sensitive to acetoacetic acid, which may be low in patients with alcoholic ketoacidosis. An assay specific for beta-hydroxybutyric acid is a more reliable marker for early evaluation of acidosis and ketosis.
Green-top Vacutainer specimen tube (may contain lithium heparin) can cause marked elevation (up to 6–8 mEq/L).
Procainamide, quinidine can produce 5–15% elevation.
Diphenhydramine, verapamil, disopyramide.
Sulfhemoglobin (cross-positive ∼10% by co-oximeter); methylene blue (2-mg/kg dose gives transiently false-positive 15% methemoglobin level); hyperlipidemia (triglyceride level of 6000 mg/dL may give false methemoglobin of 28.6%).
Falsely decreased level with in vitro spontaneous reduction to hemoglobin in Vacutainer tube (∼10%/h). Analyze within 1 hour.
Cross-reacting opioids: hydrocodone, hydromorphone, monoacetylmorphine, morphine from poppy seed ingestion. Also rifampin and ofloxacin and other quinolones in different IAs. Note: Methadone, oxycodone, and many other opioids are often not detected by routine opiate screen, may require separate specific immunoassays.
Lavender-top (EDTA) Vacutainer specimen tube (15 mOsm/L); gray-top (fluoride-oxalate) tube (150 mOsm/L); blue-top (citrate) tube (10 mOsm/L); green-top (lithium heparin) tube (theoretically, up to 6–8 mOsm/L).
Falsely normal if vapor pressure method used (alcohols are volatilized).
Many false-positives reported: chlorpromazine, dextromethorphan, diphenhydramine, doxylamine, ibuprofen, imipramine, ketamine, meperidine, methadone, thioridazine, tramadol, venlafaxine.
Phenothiazines (urine), diflunisal, ketosis,c salicylamide, accumulated salicylate metabolites in patients with renal failure (∼10% increase).
Acetaminophen (slight salicylate elevation).
Decreased or altered salicylate level: bilirubin, phenylketones.
Tetrahydrocannabinol (THC, marijuana)
Pantoprazole, efavirenz, riboflavin, promethazine, nonsteroidal anti-inflammatory drugs (depending on the immunoassay).
Caffeine overdose; accumulated theophylline metabolites in renal failure.