CA 19-9
Carbohydrate Antigen 19-9
What it is
CA 19-9 (carbohydrate antigen 19-9) is a tumour-associated carbohydrate antigen expressed on the surface of epithelial cells and shed into the circulation. It is detected using the monoclonal antibody 1116-NS-19-9, originally raised against a human colorectal cancer cell line. CA 19-9 is structurally related to the Lewis blood group antigen sialyl-Lewis A — a critical limitation of its use: approximately 5–10% of the general population does not express the Lewis (a/b) blood group antigens due to genetic variants in the fucosyltransferase gene (FUT3), and these individuals cannot express CA 19-9 regardless of any malignancy. In these Lewis-negative individuals, CA 19-9 will be undetectable (essentially zero) even in the presence of advanced cancer, making CA 19-9 uninterpretable as a screening marker. Lewis antigen status can be determined with a simple blood group test. In Lewis-positive individuals, CA 19-9 is elevated in pancreatic cancer (the most important clinical context), cholangiocarcinoma, gallbladder cancer, gastric cancer, and colorectal cancer. Benign causes of elevation include pancreatitis, cholestasis (any cause of bile duct obstruction), liver disease, and inflammatory bowel disease.
Why we measure it
Pancreatic cancer has one of the highest case fatality rates of any solid tumour — five-year overall survival is approximately 12% — primarily because it is almost invariably detected at a late, surgically unresectable stage. Symptoms (abdominal pain, weight loss, jaundice, new-onset diabetes) typically appear only after the cancer has grown substantially or obstructed adjacent structures. CA 19-9 is the most validated blood marker for pancreatic cancer, with reported sensitivity of 70–80% and specificity of approximately 90% for symptomatic pancreatic cancer. Its sensitivity for early-stage disease is substantially lower — approximately 50–60% for resectable stage I cancers — because early tumours shed less CA 19-9. However, annual trend monitoring can detect the rising pattern that precedes overt disease. New-onset diabetes or deteriorating glycaemic control in an older adult, combined with a rising CA 19-9, is a recognised warning pattern that warrants pancreatic imaging. CA 19-9 is most informative when interpreted alongside CEA, AFP, and the metabolic markers (fasting glucose, HbA1c) elsewhere in the protocol.
Why every 12 months
Annual testing provides the trend data that gives CA 19-9 its screening utility — a value rising from 12 to 19 to 28 U/mL over three consecutive annual draws (each individually 'normal') is a pattern that a single measurement cannot reveal. Benign causes of elevation (pancreatitis, cholestasis) are typically transient — a sustained rise across consecutive annual draws is a more specific signal than a single elevated reading.
What movement means
The standard upper reference limit for CA 19-9 is 37 U/mL, though laboratory-specific ranges vary. In Lewis-negative individuals, CA 19-9 will be near zero; this should be noted and future CA 19-9 testing discontinued as a cancer marker for these individuals. Benign conditions can cause substantial elevation — CA 19-9 above 37 U/mL is common in active pancreatitis and cholangitis, which must be excluded before malignancy is considered.
Normal
< 37 U/mL
Within the standard reference range. Trend across annual draws is more informative than any single reading. Individuals with a result near zero should confirm Lewis antigen status — non-expressors will always have near-zero CA 19-9.
Standard laboratory reference ranges — vary by assay method
Elevated
37 – 1,000 U/mL
Above the reference limit. Benign causes (pancreatitis, cholestasis, liver disease, IBD) are common in this range and must be assessed clinically before imaging investigation. A sustained rise across consecutive annual draws, or elevation without an identified benign cause, warrants pancreatic imaging (CT or MRI) and specialist review.
Goonetilleke KS, Siriwardena AK, European Journal of Surgical Oncology, 2007 — doi:10.1016/j.ejso.2006.10.013
Significantly Elevated
> 1,000 U/mL
At very high levels, the pre-test probability for pancreatic malignancy or other biliopancreatic cancer is substantially elevated, particularly in the absence of acute pancreatitis or biliary obstruction. Prompt specialist referral and imaging are recommended.
Goonetilleke KS, Siriwardena AK, European Journal of Surgical Oncology, 2007 — doi:10.1016/j.ejso.2006.10.013
In the protocol
References
- 1.
Goonetilleke KS, Siriwardena AK. “Systematic Review of Carbohydrate Antigen (CA 19-9) as a Biochemical Marker in the Diagnosis of Pancreatic Cancer.” European Journal of Surgical Oncology. 2007.
- 2.
Tempero MA, Uchida E, Takasaki H, Burnett DA, Steplewski Z, Pour PM. “Relationship of Carbohydrate Antigen 19-9 and Lewis Antigens in Pancreatic Cancer.” Cancer Research. 1987.
- 3.
Poruk KE, Gay DZ, Brown K, et al.. “The Clinical Utility of CA 19-9 in Pancreatic Adenocarcinoma: Diagnostic and Prognostic Updates.” Current Molecular Medicine. 2013.
- 4.
Locker GY, Hamilton S, Harris J, et al.. “ASCO 2006 Update of Recommendations for the Use of Tumor Markers in Gastrointestinal Cancer.” Journal of Clinical Oncology. 2006.