Purpose comes before interpretation

A circulating tumor marker is an analyte measured in blood, usually by an automated sandwich immunometric immunoassay. The assay measures concentration or signal under its stated conditions. What the value may mean depends on why it was ordered, the population, timing, tumor biology, clearance, and competing benign causes. The method does not measure cancer directly.

Most markers are not accurate enough for general screening of asymptomatic people because they can miss cancer and create false positives. In at-risk adults, AASLD pairs AFP with ultrasound about every six months for hepatocellular carcinoma surveillance; AFP of at least 20 ng/mL or a rising AFP prompts diagnostic multiphase imaging rather than a diagnosis. PSA screening likewise requires evidence and shared clinical decision-making, not marker availability alone. A screening program has different evidence requirements from monitoring a known marker-producing tumor.

Before reading a result, define its purpose: screening asks whether testing improves outcomes in a defined population, diagnosis support contributes one clue, prognosis estimates risk, therapy selection addresses a biomarker-linked treatment question, monitoring follows response, and recurrence surveillance follows a treated disease. CEA can be useful serially after curative-intent colorectal treatment when it was elevated before treatment, while a normal CEA does not exclude recurrence. Document the intended use before treating a reference-limit flag as meaningful.