CA 19-9 Test
CA 19-9 measures a sialylated Lewis A antigen shed by pancreaticobiliary and gastrointestinal epithelia. It is the principal tumour marker for pancreatic ductal adenocarcinoma, used to monitor therapy response, detect recurrence, and inform prognosis. Elevations occur in benign biliary obstruction and pancreatitis; Lewis-negative individuals cannot synthesise the antigen.
1
Biomarkers
Blood (serum)
Sample
2–4 business days
Turnaround
No
Fasting
What is the CA 19-9 test?
The CA 19-9 Test measures serum concentration of a sialylated carbohydrate antigen expressed and shed by epithelial cells of the pancreaticobiliary and gastrointestinal tracts. It is not a diagnostic test for cancer — tissue diagnosis remains the standard — but it is the most established tumour marker for pancreatic ductal adenocarcinoma, ordered to supplement imaging in patients with suspected pancreatic or biliary malignancy, to gauge prognosis at diagnosis, and to monitor response to chemotherapy or detect recurrence after curative-intent surgery. Practitioners order CA 19-9 when imaging suggests a pancreatic mass, when investigating obstructive jaundice of uncertain cause, and at intervals during and after treatment of known pancreatic cancer. Because benign biliary obstruction, acute and chronic pancreatitis, cirrhosis, and cholangitis can all raise CA 19-9, the result must be read in the context of imaging, bilirubin, liver enzymes, and histology. A rising trend on serial measurement carries more weight than a single elevated value, and a normal CA 19-9 does not exclude malignancy, particularly in Lewis-negative individuals who cannot produce the antigen.
What does the CA 19-9 test measure?
CA 19-9 is a sialylated Lewis A blood-group antigen expressed on the surface of epithelial cells and shed into the circulation. It is synthesised via fucosyltransferases whose activity depends on the Lewis blood-group genotype; individuals who are Lewis-antigen-negative (approximately 5–10 per cent of the population) cannot synthesise CA 19-9 and will have undetectable values regardless of disease state. Physiologically, low concentrations are shed by biliary, pancreatic, gastric, and colonic epithelia. Serum concentrations rise when tumour cells overexpress or shed the antigen or when biliary obstruction impairs clearance.
CA 19-9 is the most widely used serum marker for pancreatic ductal adenocarcinoma, where it aids prognostication, monitoring of therapy, and surveillance for recurrence after resection. It is not sufficiently sensitive or specific for screening asymptomatic populations but complements imaging in the diagnostic work-up of suspected pancreatic or biliary malignancy. Elevations occur in other gastrointestinal cancers (cholangiocarcinoma, gastric, colorectal) and in benign biliary obstruction, pancreatitis, and chronic liver disease, so interpretation must be anchored to imaging and histology. Serial trends are more informative than isolated values.
Why is the CA 19-9 test ordered?
- Evaluation of a pancreatic mass identified on CT or MRI, to complement imaging and guide biopsy decisions
- Investigation of obstructive jaundice when imaging shows biliary or pancreatic abnormality and malignancy is in the differential
- Baseline measurement before initiation of chemotherapy for pancreatic adenocarcinoma, to establish a reference for subsequent monitoring
- Serial monitoring during chemotherapy for pancreatic or biliary malignancy, to assess biochemical response alongside imaging
- Surveillance after surgical resection of pancreatic cancer, typically every 3–6 months, to detect biochemical recurrence before clinical or radiological progression
- Assessment of prognosis at diagnosis, as very high pretreatment CA 19-9 is associated with advanced disease and poorer outcomes
- Suspected cholangiocarcinoma, where CA 19-9 may be elevated but is interpreted with imaging and tissue confirmation
- Monitoring of mucinous cystic neoplasms of the pancreas when serial trends inform surveillance strategy
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
2–4 business days
Collection notes
Phlebotomy at the processing laboratory's draw site (LabCorp, Quest, or Access Medical Laboratories, depending on routing); no special preparation required. Document concurrent bilirubin and liver enzymes to aid interpretation if biliary obstruction is present or suspected.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). CA 19-9 is stable refrigerated for several days and frozen for months. Note the performing laboratory and assay platform for serial comparability, as reference limits and assay calibrations differ.
What can affect CA 19-9 results?
Individuals lacking Lewis A and Lewis B antigens (approximately 5–10 per cent, more common in certain populations) cannot synthesise CA 19-9, resulting in undetectable values regardless of disease; genotyping or Lewis-antigen phenotyping is rarely performed clinically but should be suspected when CA 19-9 is undetectable despite confirmed malignancy.
Impaired biliary clearance causes CA 19-9 to accumulate in serum; values can reach thousands in benign obstruction from choledocholithiasis or stricture, so the result must be reinterpreted after relief of obstruction if the cause is uncertain.
Inflammatory damage and ductal obstruction raise CA 19-9, often to several hundred units per millilitre; the elevation typically resolves as inflammation subsides, distinguishing pancreatitis from malignancy by serial measurement and imaging.
Reduced hepatic clearance and cholestasis can modestly raise CA 19-9, so baseline liver function and bilirubin should be documented at interpretation.
CA 19-9 is not pancreas-specific — elevations occur in cholangiocarcinoma, gastric, colorectal, and ovarian cancers, among others, so the clinical and imaging context determines the source.
Cyst-fluid CA 19-9 concentration (measured on aspirate from pancreatic cysts) can help differentiate mucinous from serous lesions, but serum CA 19-9 reflects systemic shedding and is not a direct cyst marker.
What do CA 19-9 results mean?
Elevated
Values above 37 U/mL warrant correlation with imaging and clinical context. Modest elevations (40–100 U/mL) occur in benign biliary obstruction, pancreatitis, and chronic liver disease. Moderate to high values (100–1,000 U/mL) raise concern for malignancy but overlap with benign causes; tissue confirmation is essential. Very high values (>1,000 U/mL) are more specific for advanced pancreatic or biliary cancer, often indicating unresectable or metastatic disease. A rising trend on serial testing is more concerning than a single elevated result.
Normal
A CA 19-9 within the reference range does not exclude pancreatic cancer — approximately 10–15 per cent of patients with resectable disease have normal values at diagnosis, either because they are Lewis-negative or because tumour burden or biology does not produce significant antigen shedding. Imaging and histology remain the diagnostic standards.
Reduced
Not clinically meaningful in isolation. Declining CA 19-9 during chemotherapy suggests biochemical response, particularly when accompanied by radiological stability or improvement. A fall of ≥50 per cent from baseline is often used as a surrogate endpoint in trials.
Note: CA 19-9 is a complementary marker, not a screening or stand-alone diagnostic test. Lewis-negative individuals (undetectable CA 19-9) cannot be monitored with this marker. Benign biliary and pancreatic disease can produce marked elevations, so tissue diagnosis and imaging are mandatory. Serial trends and correlation with radiological response guide therapy decisions more reliably than absolute values. Bilirubin and liver enzymes should be measured concurrently to interpret elevations in the context of biliary obstruction.
Frequently asked questions
No. CA 19-9 lacks the sensitivity and specificity for screening asymptomatic populations. It is used to complement imaging in patients with suspected pancreatic masses, to monitor therapy, and to detect recurrence after surgery.
How to access the CA 19-9 test
The CA 19-9 reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Tempero MA, et al. NCCN clinical practice guidelines in oncology: pancreatic adenocarcinoma. (2023)
- Duffy MJ, et al. Tumour markers in pancreatic cancer: a European Group on Tumor Markers (EGTM) status report. Ann Oncol. (2010)
- Ballehaninna UK, Chamberlain RS. The clinical utility of serum CA 19-9 in the diagnosis, prognosis and management of pancreatic adenocarcinoma: an evidence-based appraisal. J Gastrointest Oncol. (2012)
- Goonetilleke KS, Siriwardena AK. Systematic review of carbohydrate antigen (CA 19-9) as a biochemical marker in the diagnosis of pancreatic cancer. Eur J Surg Oncol. (2007)
- Luo G, et al. CA19-9 is a significant predictor of the postresection survival of pancreatic cancer patients: a systemic review and meta-analysis. Mol Clin Oncol. (2015)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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