|AIDS||Acquired immunodeficiency syndrome|
|CBC||Complete blood cell count|
|CHF||Congestive heart failure|
|CNS||Central nervous system|
|Diff||Differential cell count|
|EDTA||Ethylenediaminetetraacetic acid (edetate)|
|ELISA||Enzyme-linked immunosorbent assay|
|HLA||Human leukocyte antigen|
|INR||International Normalized Ratio|
|MRI||Magnetic resonance imaging|
|NPO||Nothing by mouth (nil per os)|
|PCR||Polymerase chain reaction|
|PMN||Polymorphonuclear neutrophil (leukocyte)|
|PO||Orally (per os)|
|RBC||Red blood cell|
|RPR||Rapid plasma reagin (syphilis test)|
|SIADH||Syndrome of inappropriate antidiuretic hormone (secretion)|
|SLE||Systemic lupus erythematosus|
|VDRL||Venereal Disease Research Laboratory (syphilis test)|
|WBC||White blood cell|
The clinician's main task is to make reasoned decisions about patient care despite incomplete clinical information and uncertainty about clinical outcomes. Although data elicited from the history and physical examination are often sufficient for making a diagnosis or for guiding therapy, more information may be required. In these situations, clinicians often turn to diagnostic tests for help.
When used appropriately, diagnostic tests can be of great assistance to the clinician. Tests can be helpful for screening, ie, to identify risk factors for disease and to detect occult disease in asymptomatic persons. Identification of risk factors may allow early intervention to prevent disease occurrence, and early detection of occult disease may reduce disease morbidity and mortality through early treatment. Blood pressure measurement is recommended for preventive care of asymptomatic low risk adults. Screening for breast, cervix, and colon cancer is also recommended, whereas screening for prostate cancer and lung cancer remains controversial. Optimal screening tests should meet the criteria listed in Table 1–1.
Table 1–1. Criteria for Use of Screening Procedures.
| Save Table
Table 1–1. Criteria for Use of Screening Procedures.
|Characteristics of population|
| 1. Sufficiently high prevalence of disease.|
| 2. Likely to be compliant with subsequent tests and treatments.|
|Characteristics of disease|
| 1. Significant morbidity and mortality.|
| 2. Effective and acceptable treatment available.|
| 3. Presymptomatic period detectable.|
| 4. Improved outcome from early treatment.|
|Characteristics of test|
| 1. Good sensitivity and specificity.|
| 2. Low cost and risk.|
| 3. Confirmatory test available and practical.|
Tests can also be helpful for diagnosis, ie, to help establish or exclude the presence of disease in symptomatic persons. Some tests assist in early diagnosis after onset of symptoms and signs; others assist in developing a differential diagnosis; others help determine the stage or activity of disease.
Tests can be helpful in patient management: (1) to evaluate the severity of disease, (2) to estimate prognosis, (3) to monitor the course of disease (progression, stability, or resolution), (4) to detect disease recurrence, and (5) to select drugs and adjust therapy.
When ordering diagnostic tests, clinicians should weigh the potential benefits against the potential costs and adverse effects. Some tests carry a risk of morbidity or mortality—eg, cerebral angiogram leads to stroke in 0.5% of cases. The potential discomfort associated with tests such as colonoscopy may deter some patients from completing a diagnostic work-up. The result of a diagnostic test may mandate additional testing or frequent follow-up, and the patient may incur significant cost, risk, and discomfort during follow-up procedures.
Furthermore, a false-positive test may lead to incorrect diagnosis or further unnecessary testing. Classifying a healthy patient as diseased based on a falsely positive diagnostic test can cause psychological distress and may lead to risks from unnecessary or inappropriate therapy. A screening test may identify disease that would not otherwise have been recognized and that would not have affected the patient. For example, early-stage prostate cancer detected by prostate-specific antigen (PSA) screening in a 76-year-old man with known congestive heart failure will probably not become symptomatic during his lifetime, and aggressive treatment may result in net harm.
The costs of diagnostic testing must also be understood and considered. Total costs may be high, or cost-effectiveness may be unfavorable. Even relatively inexpensive tests may have poor cost-effectiveness if they produce very small health benefits.
Factors adversely affecting cost-effectiveness include ordering a panel of tests when one test would suffice, ordering a test more frequently than necessary, and ordering tests for medical record documentation only. The operative question for test ordering is, "Will the test result affect patient management?" If the answer is no, then the test is not justified. Unnecessary tests generate unnecessary labor, reagent, and equipment costs and lead to high health care expenditures.
Molecular and genetic testing is becoming more readily available, but its cost-effectiveness and health outcome benefits need to be carefully examined. Diagnostic genetic testing based on symptoms (eg, testing for fragile X in a boy with mental retardation) differs from predictive genetic testing (eg, evaluating a healthy person with a family history of Huntington disease) and from predisposition genetic testing, which may indicate relative susceptibility to certain conditions or response to certain drug treatment (eg, BRCA-1 or HER-2 testing for breast cancer). The outcome benefits of many new pharmacogenetic tests have not yet been established by prospective clinical studies; eg, there is insufficient evidence that genotypic testing for warfarin dosing leads to outcomes that are superior to those using conventional dosing algorithms, in terms of reduction of out-of-range INRs. Other testing (eg, testing for inherited causes of thrombophilia, such as factor V Leiden, prothrombin mutation, etc) has only limited value for treating patients, ...