Imprecise usage of the terms "sensitivity" and "specificity" produces confusion in the diagnostic use of sophisticated laboratory test results. "Analytical sensitivity" represents the smallest amount of substance in a sample that can accurately be measured by an assay. "Analytical specificity" refers to the ability of an assay to measure on particular organism or substance, rather than others, in a sample. An assay's analytical sensitivity and analytical specificity are distinct from that assay's clinical diagnostic sensitivity and diagnostic specificity. "Diagnostic sensitivity" is the percentage of persons who have a given disorder who are identified by the assay as positive for the disorder. High analytical sensitivity does not guarantee acceptable diagnostic sensitivity. "Diagnostic specificity" is the percentage of persons who do not have a given condition who are identified by the assay as negative for the condition. False-positive reactions occur because of sample contamination and diminish the diagnostic specificity of the assay. The terms "sensitivity" and "specificity" should be used with the requisite adjectives because the "diagnostic" and the "analytical" meanings of these terms are very different.