Management of Stool Occult Blood False Positives
When a patient has a false positive stool occult blood test result, colonoscopy is still required to definitively rule out colorectal neoplasia before considering other causes of the false positive result. 1
Understanding False Positives in Stool Occult Blood Testing
False positive results in guaiac-based fecal occult blood tests (gFOBT) can occur due to several factors:
- Recent consumption of red meat, poultry, fish, and some raw vegetables 2
- Medications including nonsteroidal anti-inflammatory drugs (NSAIDs) and aspirin 2
- Vitamin C supplements exceeding 250 mg 2
- Hematuria contaminating stool samples collected from toilet bowls 3
- Improper test development and interpretation in laboratories 2
Evaluation Algorithm for Positive FOBT Results
Step 1: Confirm Test Validity
- Verify that the test was performed properly with 3 stool samples obtained at home 2
- Ensure patient followed pre-test instructions regarding diet and medication restrictions 2
- Note: A single-stool sample collected after digital rectal exam in the office is not an acceptable screening test 2
Step 2: Proceed with Colonoscopy
- Any positive stool occult blood test must be followed up with colonoscopy, regardless of suspicion of false positive 2, 1, 4
- Do not repeat the stool test or follow up with non-colonoscopy tests as these approaches are inappropriate 2, 1
- Do not attribute positive results to hemorrhoids without proper evaluation 1
Step 3: If Colonoscopy is Negative
- Consider upper endoscopy (EGD) if occult bleeding persists to evaluate for upper GI tract sources 1
- For patients with iron deficiency anemia and negative colonoscopy/EGD, consider capsule endoscopy 1
- Evaluate for non-GI sources of false positives:
- Hematuria (collect stool samples properly to avoid urinary contamination) 3
- Medication use (review and adjust if appropriate)
- Dietary factors (provide education on proper dietary restrictions before future testing)
Patient Education and Follow-up
- Explain the importance of proper test collection technique to avoid contamination 3
- Educate on dietary and medication restrictions prior to future testing 2
- Emphasize that annual testing is necessary for effective colorectal cancer screening 2
- If the patient had a false positive but negative colonoscopy, they can return to routine screening schedule (colonoscopy every 10 years or other appropriate screening method) 2
Important Cautions
- Never dismiss a positive FOBT without colonoscopy, as this may delay diagnosis of colorectal cancer 1, 5
- Studies show that patients with false negative FOBTs who later develop colorectal cancer may have higher mortality rates, underscoring the importance of thorough evaluation 5
- Nearly one in three physicians inappropriately repeat FOBT if the first test is positive, delaying proper diagnostic evaluation 1
By following this structured approach, clinicians can ensure appropriate follow-up of positive stool occult blood tests while minimizing unnecessary testing and potential delays in diagnosis of significant gastrointestinal pathology.