CA 19-9
Cancer antigen 19-9 — best known for pancreatic cancer follow-up. Often elevated in biliary obstruction or pancreatitis without cancer. About 5–10% of people cannot produce it at all.
What it measures
CA 19-9 is a Lewis-blood-group-related carbohydrate antigen shed by pancreatic, biliary and gastrointestinal epithelial cells. Most clinically established use: monitoring known pancreatic adenocarcinoma — pre-operative baseline + post-resection trend + chemotherapy response. NOT a screening test — far too many benign causes (cholestasis, choledocholithiasis, pancreatitis, cirrhosis, cholangitis, IBD, even smoking) elevate it. About 5–10% of people are Lewis-antigen-negative and produce no CA 19-9 at any cancer stage — the assay is uninformative for them. For incidental elevations in asymptomatic people without known risk factors, the rate of false positives is overwhelming.
What a high value can mean
- Pancreatic adenocarcinoma — the canonical indication; levels >1000 U/mL strongly suggest advanced disease.
- Cholangiocarcinoma, gallbladder cancer — biliary-tract cancers.
- Cholestasis from any cause — gallstones, primary sclerosing cholangitis, post-ERCP, hepatitis; CA 19-9 normalises when the obstruction clears.
- Acute and chronic pancreatitis — non-malignant elevation.
- Gastric, colorectal, hepatocellular cancers — less specific.
- Smoking, mucinous lung tumours, ovarian cancer — occasional elevators.
What a low value can mean
- Low or normal CA 19-9 does NOT rule out cancer — especially in Lewis-antigen-negative individuals (5–10% of the population), the test is uninformative.
When to discuss with a doctor
CA 19-9 belongs in the conversation with a gastroenterologist or oncologist — almost never a standalone result for a patient to interpret. A markedly elevated CA 19-9 in someone with painless jaundice and weight loss is concerning for pancreatic head cancer and merits urgent evaluation. Mild elevations with cholestasis or pancreatitis typically resolve when the underlying inflammation does.