The general agreement in the literature is that oral iron supplementation does not cause a false-positive guaiac-based fecal occult blood test (GFOBT).
GFOBT is based on rapid oxidization of α-guaiaconic acid to “guaiacum blue”, with hemoglobin serving as a catalyst through a non-enzymatic or “pseudoperoxidase” action. Although in vitro Fe3+ may serve as an oxidizing agent, this reaction is possible only under acidic conditions not found in the stool (pH ≥ 6-7)1. Also, in the absence of a catalyst, Fe3+ alone would not be expected to cause rapid (within 30 seconds) conversion of α-guaiaconic acid to guaiacum blue1.
Although a number of earlier clinical studies reported false-positive GBFOBT because of oral iron supplementation, subsequent investigations have uniformly failed to confirm these findings2. Potential reasons for earlier false-positive GBFOBT results include false interpretation of the color change—eg, green instead of blue— particularly when the discoloration is weakly positive, and non-standardized method of stool collection with the possibility of stool sample contamination by toilet water.
Other fascinating facts: Did you know that guaiac plant extract was used for centuries for treatment of syphilis and that the earliest application of guaiac testing was in forensic medicine in 1800s?
- McDonnell WM, Ryan JA, Seeger DM, Elta GH. Effect of iron on the guaiac reaction. Gastroenterology. 1989 Jan;96(1):74-8. https://www.ncbi.nlm.nih.gov/pubmed/2909440
- Anderson GD, Yellig TR, Krone RE. An investigation into the effects of oral iron supplementation on in vivo hemoccult stool testing. Am J Gastroenterol 1990;85:558-561. https://www.ncbi.nlm.nih.gov/pubmed/218661
Contributed by Brian Li, Medical Student, Harvard Medical School