Unit Converter
CA 19-9 (Carbohydrate Antigen 19-9)
(Sialyl-Lewisᵃ Antigen – Tumor Marker for Pancreaticobiliary Cancers)
Synonyms
- CA 19-9
- Carbohydrate Antigen 19-9
- Sialyl-Lewis A antigen
- Pancreatic cancer marker
- GI tumor marker
Units of Measurement
- U/mL (Units per milliliter - standard)
- kU/L (kilo-units per liter)
1 U/mL = 1 kU/L
Description
CA 19-9 is a tumor-associated carbohydrate antigen (sialyl-Lewisᵃ epitope) expressed on epithelial cells of the:
- Pancreas
- Biliary tract
- Stomach
- Colon
The primary clinical role of CA 19-9 is in pancreatic adenocarcinoma, specifically for:
- Treatment monitoring
- Recurrence detection
- Assessing resectability
- Prognosis estimation
It is NOT a screening test for normal-risk individuals.
Physiological Source
Low-level expression occurs in:
- Pancreatic ductal cells
- Bile duct epithelium
- Gastric/colonic epithelium
- Salivary glands
Upregulated in malignancy and cholestasis.
Clinical Significance
Elevated CA 19-9
1. Pancreatic Adenocarcinoma - Most Important
- CA 19-9 > 1000 U/mL → highly suggestive of advanced pancreatic cancer
- Used to:
- Evaluate resectability
- Monitor chemotherapy response
- Detect recurrence
- Evaluate resectability
CA 19-9 is elevated in 70–90% of pancreatic cancers.
2. Cholangiocarcinoma / Biliary Tract Cancer
Levels often high.
3. Gastrointestinal Cancers
Moderate elevation in:
- Gastric cancer
- Colorectal cancer
- Hepatocellular carcinoma
4. Benign Hepatobiliary Conditions
Often cause mild-to-moderate elevations:
- Obstructive jaundice
- Gallstones
- Cholangitis
- Pancreatitis
- Cirrhosis
⚠️ Cholestasis can raise CA 19-9 to >1000 U/mL, mimicking cancer.
5. Lung & Gynecologic Cancers
Occasional rise.
Low or Undetectable CA 19-9
Not clinically significant unless genetically absent.
Lewis Antigen–Negative Phenotype
5–10% of the population lacks Lewis a/b antigen and cannot produce CA 19-9 even in cancer.
→ false-negative results possible.
Reference Intervals
(Tietz 8E + NCCN + Mayo + ARUP)
Normal Range
- < 37 U/mL (most laboratories)
Interpretation
- 37–100 U/mL → mild elevation (often benign)
- > 100 U/mL → suspicious, especially with imaging findings
- > 300 U/mL → higher specificity for malignancy
- > 1000 U/mL → highly predictive of unresectable pancreatic cancer
(Except in cholestasis)
Prognosis
High pretreatment CA 19-9 → poorer survival.
Unit Meanings
| Unit | Meaning |
| U/mL | Units per milliliter |
| kU/L | kilo-units per liter |
Diagnostic Uses
1. Pancreatic Cancer
- Evaluate baseline tumor burden
- Monitor treatment response
- Detect recurrence post-surgery
- Assess resectability (very high CA 19-9 suggests non-resectability)
2. Biliary Tract Cancer
CA 19-9 supports diagnosis and monitoring.
3. Differentiation of Benign vs Malignant Jaundice
Not perfect, but:
- Very high CA 19-9 with mild bilirubin elevation → likely cancer
- High bilirubin + high CA 19-9 → may normalize after biliary drainage
4. Monitoring GI Malignancies
Useful in:
- Gastric cancer
- Colon cancer
- Liver cancer (adjunct to AFP)
5. Cystic Pancreatic Lesions
Elevated CA 19-9 may indicate malignant potential.
Analytical Notes
- Immunoassay based
- High bilirubin can elevate CA 19-9 → recheck after biliary drainage
- Always interpret with imaging (CT/MRI/EUS)
- Use the same assay for serial monitoring
- Hemolysis minimal effect
Clinical Pearls
- Not a screening test for pancreatic cancer due to many false positives.
- CA 19-9 may normalize after relieving biliary obstruction even if cancer is present.
- Lewis-negative individuals will never produce CA 19-9, even with pancreatic cancer.
- A rising trend is more important than a single value.
- CA 19-9 cannot distinguish benign pancreatitis from cancer without imaging.
Interesting Fact
CA 19-9 is derived from the sialylated Lewisᵃ blood group antigen, explaining why individuals lacking this antigen genotype cannot express CA 19-9.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Tumor Markers.
- NCCN Guidelines - Pancreatic Adenocarcinoma.
- ASCO - Tumor Marker Recommendations.
- IFCC - Tumor Marker Standardization.
- Mayo Clinic Laboratories - CA 19-9.
- ARUP Consult - Pancreatic Cancer Markers.
- MedlinePlus / NIH - Tumor Marker Overview.