The question of when a certain concentration of a serum biomarker turns into a tumor marker, i. e. an objectively verifiable criterion for tumor diagnosis, leads to the analysis of the specificity (a measure of the proportion of correctly identified healthy individuals), sensitivity (a measure of the proportion of correctly recognized cancer patients), and precision (positive predictive value). Or in short: a tumor marker is specific if no healthy individual has one. This constitutes the problem: The serum concentrations of biomarkers of cancer patients and healthy subjects overlap. Healthy individuals occasionally have "tumor markers" and cancer patients in turn sometimes express inconspicuous biomarkers.
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