What the result supports and what it cannot establish

A tumor marker may support a clinical question, but an isolated abnormal value rarely establishes cancer. Population screening requires evidence that the program improves outcomes and has acceptable harms, not simply that an assay is available. PSA screening guidance is population and age dependent, and AFP surveillance in an at-risk population is paired with ultrasound and diagnostic imaging when recall criteria are met. Keep a reference limit separate from a clinical decision threshold.

For serial follow-up, ask whether the same assay and laboratory were used, whether the marker was informative before treatment, and whether biologic timing fits the trend. ASCO colorectal follow-up uses CEA at intervals after curative-intent treatment, yet a normal CEA does not exclude recurrence and a substantial rise should prompt confirmation and imaging. ATA guidance treats imaging as primary surveillance when TgAb is present, with serial TgAb trend as an ancillary signal. A trend is evidence, not a stand-alone diagnosis.

Do not recommend screening outside evidence-based programs, prescribe a universal dilution reflex, or use universal reference intervals. Keep guidance current with guidelines, approvals, local analyzer or reagent IFUs, local policy, and interference information. Release tumor-marker results with method continuity and uncertainty visible, then route discordance through the local laboratory pathway.