Which factor may compromise the accuracy of the Urea breath test or fecal antigen test for H. pylori?

• A. Metronidazole
• B. Amoxicillin
• C. Bismuth subsalicylate
• D. Omeprazole

Answer: (D)

Ongoing suppression of gastric acid with a proton pump inhibitor like omeprazole can lower the density and activity of Helicobacter pylori in the stomach. The urea breath test relies on H. pylori urease to split ingested labeled urea into CO2; if the bacteria are suppressed, urease activity drops, producing less labeled CO2 and increasing the chance of a false-negative result even if infection is still present. The fecal antigen test detects bacterial antigens shed into stool; with reduced bacterial load under PPI therapy, fewer antigens are shed, which can also yield a false-negative result. Stop the PPI for an appropriate interval before testing to avoid this issue. While antibiotics or bismuth can affect results by lowering bacterial numbers, the most consistent way PPIs compromise accuracy is through reduced bacterial activity and load.