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
CA 19-9 is a carbohydrate antigen measured in blood, studied mainly in relation to pancreatic and biliary conditions. Much of that research focuses on pancreatic cancer, which is usually found late: five-year survival is about 12%, and symptoms (abdominal pain, weight loss, jaundice, new-onset diabetes) tend to appear only at an advanced stage. Studies report CA 19-9 sensitivity of 70–80% and specificity of about 90% in people with symptoms, but lower sensitivity (about 50–60%) at early stages — which is why research gives more weight to a rising trend than to any single value. Studies have also highlighted new-onset diabetes or worsening glucose control in older adults, alongside a rising CA 19-9, as a pattern worth medical attention. In the Blueprint protocol, CA 19-9 is read alongside CEA, AFP, and the metabolic markers (fasting glucose, HbA1c). It is not diagnostic on its own — pancreatitis, bile duct blockage, and other conditions also raise it.
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; research notes that CA 19-9 is not informative 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. A result near zero may reflect Lewis antigen status — people who do not express it 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. Non-cancer causes (pancreatitis, bile duct blockage, liver conditions, IBD) are common in this range. Research gives more weight to a sustained rise across consecutive draws. CA 19-9 is not diagnostic on its own. Discuss with a registered medical practitioner.
Goonetilleke KS, Siriwardena AK, European Journal of Surgical Oncology, 2007 — doi:10.1016/j.ejso.2006.10.013
Significantly Elevated
> 1,000 U/mL
Markedly above range. Studies report that the likelihood of a significant pancreatic or biliary cause rises at this level, particularly without acute pancreatitis or bile duct blockage. CA 19-9 is not diagnostic on its own. Discuss with a registered medical practitioner.
Goonetilleke KS, Siriwardena AK, European Journal of Surgical Oncology, 2007 — doi:10.1016/j.ejso.2006.10.013
This page summarises published research for general education. It is not medical advice and does not interpret individual results. Discuss your results with a registered medical practitioner.
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.