Cancer Antigen 19-9
Tumor MarkersLast reviewed: April 7, 2026. Source approach: standard laboratory interpretation context, general medical reference material, and public-health or clinical guidance where relevant.
What is Cancer Antigen 19-9?
Cancer Antigen 19-9 (CA 19-9) is a carbohydrate antigen—specifically a sialylated Lewis-a blood group antigen—that is found on the surface of certain epithelial cells and shed into the bloodstream. It was originally identified using a monoclonal antibody (1116 NS 19-9) raised against a human colon carcinoma cell line. CA 19-9 is produced by normal pancreatic and biliary ductal cells, as well as gastric, colonic, endometrial, and salivary epithelium at low levels.
CA 19-9 is the most widely used serum biomarker for pancreatic adenocarcinoma. It serves primarily as a tool for monitoring treatment response, assessing surgical resectability, and detecting disease recurrence. CA 19-9 is not recommended as a screening test for pancreatic cancer in the general population due to insufficient sensitivity and specificity. Approximately 5–10% of the population lacks the Lewis antigen and cannot synthesize CA 19-9, making the test uninformative in these individuals regardless of disease status. Biliary obstruction from any cause can markedly elevate CA 19-9, reducing its specificity.
Why It Matters
CA 19-9 is the most clinically useful serum marker for pancreatic ductal adenocarcinoma. Preoperative CA 19-9 levels help predict resectability—very high values (>1000 U/mL) often indicate unresectable or metastatic disease. Declining CA 19-9 after surgery or chemotherapy suggests treatment response, while rising levels signal disease progression or recurrence. Because CA 19-9 is also elevated in benign biliary obstruction, cholangitis, pancreatitis, and other gastrointestinal conditions, results must be interpreted alongside imaging and clinical findings.
Normal Reference Ranges
| Group | Range | Unit |
|---|---|---|
| Adults | 0–37 | U/mL |
| Lewis antigen-negative individuals | Undetectable (test not applicable) | U/mL |
Reference ranges may vary by laboratory. Always compare results to the ranges provided by your testing facility.
What High CA 19-9 Levels Mean
Common Causes
- Pancreatic adenocarcinoma
- Cholangiocarcinoma (bile duct cancer)
- Biliary obstruction (benign or malignant)
- Acute and chronic pancreatitis
- Cholangitis
- Colorectal cancer
- Gastric cancer
- Hepatocellular carcinoma
- Cystic fibrosis
- Liver cirrhosis
Possible Symptoms
- Jaundice (yellowing of skin and eyes)
- Unexplained weight loss
- Abdominal or back pain
- New-onset diabetes in adults over 50
- Loss of appetite
- Dark urine and pale stools
- Nausea
What to do: Elevated CA 19-9 requires imaging (typically CT or MRI of the abdomen) to identify the underlying cause. If biliary obstruction is present, CA 19-9 should be re-measured after biliary drainage, as obstruction alone can cause marked elevation. For patients with known pancreatic cancer, serial CA 19-9 is used to monitor chemotherapy response and detect recurrence. Levels >1000 U/mL preoperatively may indicate unresectable disease. Additional workup may include endoscopic ultrasound with biopsy and cross-sectional imaging.
What Low CA 19-9 Levels Mean
Common Causes
- Normal physiological state
- Lewis antigen-negative phenotype (5–10% of population)
- Successful cancer treatment
Possible Symptoms
- No symptoms associated with low CA 19-9
What to do: Low or undetectable CA 19-9 is typically normal. In Lewis antigen-negative individuals, CA 19-9 will be undetectable regardless of disease status, and the test should not be used for monitoring. After cancer treatment, normalization of CA 19-9 is a favorable prognostic indicator, though continued surveillance with imaging remains essential.
When Is CA 19-9 Testing Recommended?
- Monitoring treatment response in pancreatic cancer
- Assessing surgical resectability of pancreatic tumors
- Surveillance for recurrence after pancreatic cancer surgery
- Helping differentiate malignant from benign pancreatic cysts
- Monitoring cholangiocarcinoma treatment
Frequently Asked Questions
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References and review approach
Biomarker glossary pages are educational explainers and should be interpreted alongside the reference ranges and comments provided by your laboratory and clinician. For our editorial standards and review approach, see our Editorial Policy and Content Review Process.
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Upload Lab Results →Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult your healthcare provider for interpretation of your specific test results.