A 58-year-old man presents with painless obstructive jaundice and weight loss. CT shows a mass in the head of the pancreas. Serum CA 19-9 is requested for monitoring. In a subset of patients this marker will be falsely normal because:

• A CA 19-9 levels are suppressed by concurrent obstructive jaundice
• B CA 19-9 is cleared exclusively by the kidneys and rises only after renal failure develops
• C CA 19-9 is produced only by colonic adenocarcinomas and not by pancreatic tumours
• D CA 19-9 is a sialylated Lewis blood group antigen that cannot be synthesised by Lewis antigen-negative individuals ✓

Explanation

CA 19-9 is a sialylated derivative of the Lewis D blood group antigen. About 5 to 10 percent of the population lacks the Lewis antigen phenotype and therefore cannot produce CA 19-9 even when they have advanced pancreatic cancer, giving a falsely reassuring value. Obstructive jaundice and pancreatitis actually raise CA 19-9 non-specifically rather than suppress it, which kills option A.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.