The impact of disease prevalence on the predictive value of laboratory tests in primary care.

The predictive value of any diagnostic process is related to the sensitivity and specificity of the test and the prevalence of the underlying pathology for which the diagnostic testing is accomplished. Prevalence is the most important, but least understood, factor affecting the usefulness of a test result. For each accomplished test which is not helpful in either supporting a diagnosis or assisting in a differential diagnosis, the health care cost is increased without a corresponding benefit in the value of the information obtained.