What does the Cancer Antigen 19-9 test measure?

Why is Cancer Antigen 19-9 measured?

CA 19-9 is the most widely used serum marker for pancreatic ductal adenocarcinoma, where it aids prognostication, monitoring of therapy, and surveillance for recurrence after resection. It is not sufficiently sensitive or specific for screening asymptomatic populations but complements imaging in the diagnostic work-up of suspected pancreatic or biliary malignancy. Elevations occur in other gastrointestinal cancers (cholangiocarcinoma, gastric, colorectal) and in benign biliary obstruction, pancreatitis, and chronic liver disease, so interpretation must be anchored to imaging and histology. Serial trends are more informative than isolated values.

Reference ranges

Standard reference

Typically <37 U/mL; laboratory-specific.

Functional medicine optimal

Not applicable — CA 19-9 is a tumour marker, not a health-optimisation analyte. Values remain within the reference limit (<37 U/mL) in healthy individuals and Lewis-positive patients without malignancy or biliary obstruction.

Units

U/mL, kU/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.