Beets and False Negative Results in Hemoccult Tests
Beets do not cause false negative results in hemoccult tests, but excess vitamin C (from supplements or citrus fruits) can cause false negative results in guaiac-based fecal occult blood tests (gFOBT). 1, 2
Causes of False Negative Results in Hemoccult Tests
- Excess vitamin C (more than 250 mg) from either supplements or citrus fruits and juices can result in false negative results in guaiac-based tests by blocking the peroxidase reaction 1, 2
- Low pH (below 2) can cause false negative results when testing gastric samples 3
- Progressive heme degradation during fecal storage can cause loss of reactivity in Hemoccult tests 4
- Inadequate specimen collection or improper test development and interpretation can lead to false negative results 2
What Does Not Cause False Negatives but Should Be Noted
- Beets are not mentioned in any guidelines or research as a cause of false negative results in hemoccult tests 1, 2, 1
- Red meat consumption is associated with increased false positive rates (not false negatives) 2, 5
- NSAIDs and aspirin (more than one adult aspirin per day) can increase false positive rates (not false negatives) 2, 5
Clinical Implications of False Negative Results
- False negative results can lead to missed diagnoses of colorectal cancer 6
- Patients with false negative FOBTs may have a threefold higher prevalence of metastatic disease compared to those who eventually convert to positive tests 6
- All-cause mortality is significantly higher among patients with false negative results compared to those who eventually have positive tests 6
Recommendations to Minimize False Results
- Instruct patients to avoid vitamin C in excess of 250 mg from supplements or citrus fruits for 3 days before testing 1, 2
- Ensure proper collection of 3 stool samples at home rather than a single sample collected after digital rectal exam 2, 5
- Consider using fecal immunochemical tests (FIT) instead of guaiac-based tests, as they are not affected by vitamin C and have better specificity for lower gastrointestinal bleeding 1, 5
- Ensure laboratory quality assurance procedures are followed for test development and interpretation 2
- Any positive test should be followed up with colonoscopy, not repeated stool testing 1, 2
Technical Considerations
- Hemoccult sensitivity varies widely, remaining negative with up to 42.5 mg hemoglobin per gram of stool but positive with as little as 0.04 mg of hemoglobin per gram of stool 4
- Stool liquidity affects test results - driest stools tend to be Hemoccult-negative while wettest stools tend to be Hemoccult-positive 4
- The intestinal converted fraction of hemoglobin (degraded by gut flora) is not detected by guaiac tests and is often higher in Hemoccult-negative stools 4
Remember that proper patient education about test preparation and appropriate follow-up of results are essential to maximize the effectiveness of hemoccult testing for colorectal cancer screening.