Abstract
BACKGROUND: Current laboratory tests exhibit high sensitivity and specificity combined with comparatively low costs thus favoring broad and uncritical ordering habits. METHODS: Introduction of Bayes' theorem and discussion of its implications for laboratory test results in a mostly non-technical form, accompanied by a selective literature review. RESULTS AND CONCLUSIONS: According to Bayes' theorem the positive predictive value of laboratory test results is directly dependent on the prevalence of the disease in a given patient cohort. Thus, the clinical value of a given test result is critically dependent on a precise indication. Ordering of tests that are not indicated in a given patient is clinically useless and undesirable, where detailed information on disease prevalence is missing. These considerations are valid irrespective of ethical or economic considerations.