Which dietary factor can cause a false-negative guaiac stool test result?

• A. Vitamin C
• B. Red meat
• C. Caffeine
• D. Dairy

Answer: (A)

The test relies on a peroxidase reaction where heme in any hidden blood catalyzes oxidation of guaiac, turning the reagent blue. A high intake of Vitamin C can block this reaction because it acts as an antioxidant and reducing agent, neutralizing hydrogen peroxide used in the reaction. With Vitamin C present in high amounts, the expected blue color may not develop even if occult blood is there, producing a false-negative result. In practice, patients are often advised to avoid large doses of vitamin C for a few days before testing to prevent this interference.

Red meat can cause a false-positive result because heme from meat can add peroxidase-like activity, mimicking blood. Caffeine and dairy do not reliably cause false negatives in this test.