Which supplement is associated with a false-positive fecal occult blood test?

• A. Vitamin C
• B. Vitamin B12
• C. Vitamin D
• D. Vitamin K

Answer: (A)

The test detects hidden blood by a peroxidase reaction: heme in blood helps oxidize guaiac, turning the substrate blue. Vitamin C, a strong antioxidant, can interrupt this oxidation. In high doses it scavenges the reactive species needed for the blue color, so even when blood is present you may not see the color change — a false-negative result. It is not known for causing false-positive results; substances that can lead to false positives are typically peroxidase-containing foods (like horseradish or certain vegetables) or recent red meat, not vitamins. So the idea to associate a supplement with a false-positive isn’t supported by how the guaiac-based FOBT works; vitamin C more commonly causes false negatives.