False-Positive Fecal Occult Blood Test
Dietary peroxidases (Option D) are the most likely cause of a false-positive guaiac-based fecal occult blood test.
Mechanism of False-Positive Results with Guaiac-Based Tests
The guaiac-based FOBT (gFOBT) detects blood through the pseudoperoxidase activity of heme or hemoglobin, causing a color change on guaiac-impregnated slides 1. This chemical reaction is not specific to human blood and can be triggered by any substance with peroxidase activity 1.
Dietary Peroxidases: The Primary Culprit
Red meat (beef, lamb, liver), poultry, and fish contain peroxidase activity that directly causes false-positive results by mimicking the chemical reaction of blood 1, 2.
The American Cancer Society explicitly recommends avoiding red meats for 3 days before testing because recent consumption of red meat is associated with increased false positivity 1.
Some raw vegetables also contain peroxidase activity that can trigger false-positive results 2.
Manufacturers endorse avoidance of these dietary peroxidases specifically to reduce the risk of false-positive results 1.
Why Other Options Are Less Likely to Cause False Positives
Oral Iron Therapy (Option A)
- Iron therapy does not cause false-positive gFOBT results 1.
- Iron is not mentioned in major guidelines as a cause of false-positive results.
Medications That Cause True Bleeding (Not False Positives)
Critical distinction: NSAIDs, aspirin, and anticoagulants cause true bleeding from the gastrointestinal tract, not false-positive chemical reactions 1.
Aspirin (Option C) and NSAIDs (Option E) should be avoided for 7 days prior to testing with sensitive guaiac-based tests, but this is to reduce false positivity from medication-induced bleeding, not from chemical interference 1.
Coumadin/warfarin (Option B) similarly causes actual bleeding that lowers the positive predictive value for advanced neoplasia, but this represents true blood in stool, not a false-positive chemical reaction 1.
With fecal immunochemical tests (FIT), which detect human globin specifically, aspirin and anticoagulant use actually improved sensitivity with little decrease in specificity, and there is no rationale to adjust these medications 1.
Clinical Implications
The question asks specifically about false-positive results, meaning a positive test without actual blood present 1.
Dietary peroxidases create a chemical false-positive by triggering the guaiac reaction without any blood being present 1, 3.
Medications like NSAIDs, aspirin, and warfarin cause real bleeding from non-neoplastic sources, which technically represents a true-positive test (blood is present) with low positive predictive value for cancer 1.
One study demonstrated that delaying development of Hemoccult SENSA by 3-14 days allows breakdown of ingested peroxidases, eliminating the need for dietary restrictions in populations with low red meat intake 3.