CA 19-9
Summary
CA 19-9 is a tumor marker primarily associated with pancreatic adenocarcinoma, used for monitoring disease progression and treatment response rather than screening or diagnosis. It can also be elevated in other GI malignancies (cholangiocarcinoma, colorectal, gastric cancer) and benign conditions like biliary obstruction and pancreatitis.
Detail
CA 19-9 (Cancer Antigen 19-9) is a sialylated Lewis blood group antigen carbohydrate epitope found on mucins, primarily produced by pancreatic and biliary ductal epithelium. It is the most widely used serum marker for pancreatic adenocarcinoma, with sensitivity and specificity around 70-90%, but it lacks sufficient specificity for use as a screening test due to false positives in benign hepatobiliary and pancreatic conditions (e.g., cholangitis, cholestasis, pancreatitis) and other malignancies (cholangiocarcinoma, gastric, colorectal, and ovarian mucinous tumors). Clinically, CA 19-9 is used to: (1) monitor response to therapy and detect recurrence after resection of pancreatic cancer, (2) provide prognostic information (very high levels correlate with unresectable or metastatic disease), and (3) aid in distinguishing pancreatic cancer from chronic pancreatitis in ambiguous cases, though overlap exists. Importantly, individuals who are Lewis antigen-negative (Le a-b-, approximately 5-10% of the population) cannot synthesize CA 19-9 and will have falsely low or undetectable levels even with extensive tumor burden, which is a classic board-testable pitfall. CA 19-9 should never be used as a standalone diagnostic test for pancreatic cancer; imaging (CT, MRI, EUS) and tissue biopsy remain the gold standard for diagnosis.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- UpToDate: Tumor markers in pancreatic cancer
- Goldman-Cecil Medicine
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