A patient with biopsy-proven pancreatic adenocarcinoma has a serum CA 19-9 of only 8 U/mL despite extensive disease. The most likely explanation is:
- A Tumour of the uncinate process origin
- B Laboratory assay error from haemolysis
- C Lewis antigen negative phenotype preventing CA 19-9 expression ✓
- D Concurrent biliary obstruction lowering the value
Correct answer: C. Lewis antigen negative phenotype preventing CA 19-9 expression
Explanation
CA 19-9 is a sialylated Lewis blood group antigen. About 5 to 10 percent of people are Lewis antigen negative and cannot synthesise CA 19-9, so they have falsely low values even with advanced disease, making the marker unusable for monitoring in them. Biliary obstruction and cholangitis do the opposite, causing falsely elevated values, which is why CA 19-9 should be interpreted after bilirubin normalises.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.