Causes of False Positive Results for Stool Occult Blood Tests
The most common causes of false positive stool occult blood test results include consumption of red meat, certain medications like NSAIDs and aspirin, high-dose vitamin C, and contamination of stool samples with menstrual or urinary blood. 1
Dietary Factors
- Recent consumption of red meat (beef, lamb, and liver) is strongly associated with false positive results in guaiac-based fecal occult blood tests (gFOBT) due to the pseudoperoxidase activity of animal hemoglobin 1
- Poultry, fish, and some raw vegetables may also contribute to false positive results 1
- Manufacturers recommend avoiding these foods for 3 days before testing to reduce false positivity rates 1
Medication-Related Causes
- Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and naproxen should be avoided for 7 days prior to testing 1
- Aspirin at doses higher than one adult aspirin per day can increase false positive rates (low-dose cardioprotective aspirin regimens are generally acceptable) 1
- Iron supplements can interfere with test results and should be avoided during the testing period 1
Procedural and Sample Collection Issues
- Digital rectal examination immediately before collecting stool samples can cause trauma-related bleeding, leading to false positive results 1
- Collection of stool samples from toilet bowls contaminated with blood-containing urine can cause false positive results 2
- Hematuria may not be noticed by patients or may be masked by colored toilet bowl cleaners 2
- Single-stool sample FOBT collected after digital rectal exam in the office has very poor sensitivity and higher risk of false positives 1
- Rehydration of gFOBT slides substantially increases the false positive rate and is not recommended 1
Other Factors
- Bleeding from non-colorectal sources (such as hemorrhoids, anal fissures, or menstrual blood) can lead to false positive results 3
- Laboratory variation in test interpretation can affect results - ongoing technician training and standardized laboratory procedures are important 1
- False positive rates vary by demographic factors - older patients, males, and non-White patients are significantly more likely to receive false positive results 4
Clinical Implications
- After 10 years of annual FOBT screening, approximately 23% of patients will experience at least one false positive result 4
- False positive results lead to unnecessary colonoscopies, increasing patient discomfort, costs, and risks 1
- Proper patient education about dietary restrictions and medication avoidance is essential to minimize false positive results 1
Reducing False Positive Results
- Use fecal immunochemical tests (FIT) instead of guaiac-based tests when possible, as they do not require dietary restrictions and have fewer false positives 1
- Ensure proper collection of stool samples from three consecutive bowel movements rather than a single sample 1
- Process guaiac-based tests in clinical laboratories rather than physician offices to improve standardization and reduce interpretation errors 1
- Advise patients to avoid obvious sources of contamination such as menstrual blood or urinary blood during the collection period 2, 3