A 58-year-old man presents with painless obstructive jaundice. Imaging shows a 4 cm carcinoma of the pancreatic head. His serum CA 19-9 is only 14 U/mL (upper reference limit 37 U/mL) despite the bulky tumour. The most likely explanation for this falsely reassuring value is:

• A Rapid tumour doubling time outpacing marker synthesis
• B Lewis antigen negativity, preventing synthesis of the CA 19-9 epitope ✓
• C Competitive binding of bilirubin to the assay antibody
• D Downregulation of the mucin gene MUC1 in pancreatic adenocarcinoma

Explanation

CA 19-9 is a sialylated derivative of the Lewis blood group antigen produced by ductal cells of pancreatic, biliary and gastric epithelium. About 5 to 10 percent of the population lacks the Lewis antigen and therefore cannot synthesise CA 19-9 at all, giving spuriously low levels even with advanced disease. Obstructive jaundice from any cause can also raise CA 19-9 non-specifically, which further limits the marker rather than lowering it, ruling out option C.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.