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Chapter adapted and updated, with permission, from Nicoll D et al. Guide to Diagnostic Tests, 7th ed. McGraw-Hill, 2017.

When used appropriately, diagnostic tests can be of great assistance to the clinician. Tests can be used 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 (using prostate-specific antigen [PSA]), lung cancer (in patients without a history of smoking), and thyroid cancer remains controversial. Screening without demonstrated benefits should be avoided. Optimal screening tests should meet the criteria listed in Table e2–1. Some positive screening test results (eg, rapid HIV antibody [Ab] tests) require confirmatory testing.

Table e2–1.Criteria for use of screening procedures.

Tests can be used 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 severity, activity, or stage of disease.

Tests can also be used in patient management. They can help (1) estimate prognosis and facilitate treatment decision making, (2) monitor the course of disease or treatment response (progression, stability, residual, or remission), (3) select drugs and adjust therapy, (4) detect disease recurrence, and (5) determine necessity of hospital admission and readiness for discharge.

One evolving field of medicine is personalized medicine, which involves tailoring treatment to the individual patient. Precision medicine is emerging as a tool to match patients with the appropriate treatment based on the precise biologic and molecular characteristics of an individual patient's disease (cancer, in particular). As an example, only patients with breast cancer that shows overexpression of HER2 protein or extra copies of the HER2 gene or both could benefit from trastuzumab treatment. The associated companion diagnostics for the safe and effective use of various drugs is rapidly evolving.

When ordering diagnostic tests, clinicians should weigh the potential benefits against the potential costs and adverse effects (ie, test stewardship). Some tests carry a risk of morbidity or mortality—eg, cerebral angiogram leads to stroke in 0.5% of cases. The potential ...

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