Do iron supplements interfere with stool occult blood tests?

Iron Tablets and Stool Occult Blood Testing Interference

Iron supplements should be discontinued at least 3 days before fecal occult blood testing to avoid potential false positive results, particularly with guaiac-based tests.

Types of Fecal Occult Blood Tests and Iron Interference

The relationship between iron supplements and fecal occult blood tests varies by test type:

Guaiac-Based Tests (Hemoccult)

• Traditional evidence: Older studies reported significant false positive rates with iron supplements
  • A 1982 study showed 50-65% false positive rates with ferrous sulfate and ferrous gluconate 1
  • These older studies led to the common recommendation to stop iron before testing

Immunochemical Tests (FIT)

• Modern recommendation: Fecal immunochemical tests (FIT) are less susceptible to medication interference 2
• Clinical advantage: FIT tests specifically detect human hemoglobin and are not affected by dietary factors or medications

Current Evidence on Iron Interference

The evidence regarding iron interference is actually contradictory:

• Evidence against interference: More recent controlled studies show minimal interference

  • A 1991 prospective trial found that therapeutic doses of ferrous sulfate and ferrous gluconate did not cause false-positive Hemoccult tests 3
  • A 1990 randomized, double-blind crossover study with 78 participants showed no positive results for occult blood after iron ingestion when tested by either Hemoccult II or Hemoccult Sensa methods 4

• Evidence for interference:

  • A 1995 in vitro study showed Hemoccult slides were falsely positive for solutions containing only iron 5
  • The 1982 controlled study showed high false-positive rates with both Hemoccult and Hematest methods 1

Clinical Recommendations

Before Testing

1. Discontinue iron supplements: Stop iron tablets at least 3 days before occult blood testing 2
2. Test selection: Consider using FIT tests for patients on iron therapy who cannot discontinue treatment

After Testing

1. Interpret positive results carefully: Any positive stool Hemoccult test in patients taking iron should not be automatically dismissed as false-positive 3
2. Follow-up: Positive occult blood tests in patients with iron deficiency anemia require complete evaluation with colonoscopy and upper endoscopy 2

Common Pitfalls to Avoid

• Assuming all positive tests in iron users are false positives: This could lead to missed diagnoses of significant gastrointestinal pathology
• Not considering test type: Different occult blood tests have varying susceptibility to interference
• Inadequate patient preparation: Failure to advise patients to stop iron before testing can compromise test accuracy
• Overreliance on single tests: Multiple stool samples should be tested to minimize false-negative results 6

For patients with iron deficiency anemia requiring investigation, the British Society of Gastroenterology recommends that iron therapy should not be deferred awaiting investigations unless colonoscopy is imminent 7, highlighting the importance of balancing diagnostic accuracy with therapeutic needs.