How long should a patient with iron deficiency anemia, taking ferrous (iron) supplements, stop taking the medication before undergoing a stool occult blood test to avoid false positive results?

Patients Taking Iron Supplements Do NOT Need to Stop Before Stool Occult Blood Testing

Patients on ferrous iron supplements should continue their medication before stool occult blood testing, as modern fecal immunochemical tests (FIT) and Hemoccult methods do not produce false-positive results from oral iron supplementation. 1, 2

Evidence Against Holding Iron Supplements

The concern about false-positive results from oral iron is outdated and not supported by current evidence:

• A prospective, randomized, double-blind study of 78 healthy volunteers found zero false-positive results on Hemoccult II or Hemoccult Sensa testing after taking therapeutic doses of either ferrous sulfate or ferrous gluconate for 2 weeks. 1

• A separate study using both Hemoccult testing and quantitative HemoQuant assays in 14 volunteers confirmed that ferrous sulfate (324 mg three times daily) and ferrous gluconate (324 mg three times daily) produced no false-positive results and did not increase gastrointestinal blood loss. 2

• Any positive stool occult blood test in patients taking therapeutic iron should be considered a true positive and warrants investigation, not dismissal as a false positive. 2

When Fecal Occult Blood Testing Should Be Avoided

The guidelines identify specific situations where stool testing has limited utility, but iron supplementation is not among them:

• Fecal occult blood testing should not be used to investigate iron deficiency anemia itself, as it is insensitive and non-specific for this purpose. 3, 4

• FIT testing should be avoided in patients with iron deficiency anemia when used for colorectal cancer screening (sensitivity only 58%, specificity 84%), but this is due to the underlying condition, not the iron supplements. 3

• FIT should be avoided during acute diarrhea episodes due to reduced test accuracy. 3

Important Caveats

While modern immunochemical tests are not affected by oral iron, one older study from 1982 reported false-positive rates of 50-65% with Hemoccult methods after iron ingestion. 5 However, this finding was directly contradicted by subsequent higher-quality studies using the same Hemoccult methods. 1, 2 The discrepancy likely reflects methodological differences or improvements in test manufacturing.

The bottom line: There is no need to hold iron supplements before fecal occult blood testing with modern methods, and doing so unnecessarily delays appropriate iron replacement therapy. 1, 2