Which supplement can cause a false-positive fecal occult blood test?

Which supplement can cause a false-positive fecal occult blood test?

• A. Vitamin C
• B. Vitamin E
• C. Iron
• D. Calcium

Answer: (A)

The test detects hidden blood by a peroxidase reaction: heme in blood catalyzes a reaction that turns a guaiac-impregnated paper blue. High-dose ascorbic acid is a strong reducing agent that can quench this color change, leading to a false-negative result. It doesn’t typically cause a false-positive result. If the test used is immunochemical, supplements generally don’t affect it. So the important point is that vitamin C can mask blood in the stool by preventing the color change, not produce a false positive.

The test detects hidden blood by a peroxidase reaction: heme in blood catalyzes a reaction that turns a guaiac-impregnated paper blue. High-dose ascorbic acid is a strong reducing agent that can quench this color change, leading to a false-negative result. It doesn’t typically cause a false-positive result. If the test used is immunochemical, supplements generally don’t affect it. So the important point is that vitamin C can mask blood in the stool by preventing the color change, not produce a false positive.