Laboratory tests can be valuable aids in making a diagnosis, but, as screening tools for detecting hidden disease in asymptomatic individuals, their usefulness is limited. The value of a test as a diagnostic aid depends on its sensitivity and specificity. Sensitivity is the measure of the percentage of individuals with the disease who have a positive test result (i.e., people with the disease who are correctly identified by the procedure), and specificity is the measure of the percentage of people without the disease who have a negative test result (i.e., healthy individuals correctly identified as free of the disease). If a test is 100 percent sensitive and the test result is negative, it can be said with certainty that the person does not have the disease, because there will be no false-negative results. If the test is not specific enough, however, it will yield a large number of false-positive results (positive test results for those who do not have the disease). The ideal test would be 100 percent sensitive and 100 percent specific; an example would be an early pregnancy test that was so accurate that it was positive in every woman who was pregnant and was never positive in a woman who was not pregnant. Unfortunately no such test exists. The normal value for a test is based on 95 percent of the population tested being free of disease, meaning that 1 out of every 20 test results in healthy individuals will be outside the normal range and therefore positive for the disease.

In the past, physicians would order selected tests based on the likelihood that the person had a certain disease. With the advent of automated analyzers, an increasing number and variety of tests have been made available at greatly reduced cost so that as many as 18 or more tests can be performed for what it previously cost to carry out three or four individual tests. A panel of chemical tests for blood and urine have become routine components of the basic medical workup. A disadvantage of this strategy is that each test produces some false-positive results and requires additional tests to rule out these diseases. The trend is reversing to perform only those tests most likely to be cost-effective.