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The clinician’s main task is to make reasoned decisions about patient care based on available clinical information and estimated clinical outcomes. Although data elicited from the history and physical examination may be sufficient for making a diagnosis or for guiding therapy, more information is often required. Today’s clinicians rely increasingly on diagnostic tests and face challenges in selecting which tests to order and in interpreting test results. This chapter aims to help clinicians understand the utility as well as the limitations of diagnostic testing in clinical diagnosis and management.


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, colon, and lung cancer is also recommended, whereas screening for prostate cancer remains controversial. Screening without demonstrated benefits should be avoided. Optimal screening tests should meet the criteria listed in Table 1–1. Some screening test results (eg, rapid HIV Ab tests) require confirmatory testing.

Table 1–1.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 stage or activity of disease.

Tests can also be used in patient management. They can help (1) evaluate the severity of disease, (2) estimate prognosis, (3) monitor the course of disease (progression, stability, or resolution), (4) detect disease recurrence, and (5) select drugs and adjust therapy.

One evolving field of medicine is personalized medicine, which involves tailoring treatment to the individual patient. A companion diagnostic test may be used to identify which patients could benefit from a drug and which patients would not benefit or even be harmed. 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.

When ordering diagnostic tests, clinicians should weigh the potential benefits against the potential ...

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