Stool Occult Blood Test Interactions

Vitamin C (ascorbic acid) is a reducing agent that blocks the guaiac peroxidase color reaction used in traditional fecal occult blood tests (gFOBT, including Hemoccult), producing a falsely negative result even when significant gastrointestinal bleeding is present. Doses of 250 mg per day or higher have been documented to cause false negatives, leading to missed diagnoses of colorectal bleeding sources.