“The HgB has dropped, have you checked a guaic?”

How many times have you checked a FOBT in your workup for anemia? Let’s take a look at what FOBT is supposed to be used for.

FOBT is a visual stool assay in which heme in the stool reacts with reagent hydrogen peroxide to oxidize guaiac, leading to a color change. A color change from clear to blue suggests a positive test; i.e, clear to blue means there’s blood in the poo. The FOBT was validated for one singular screen, however – Colorectal Cancer. It has never been validated for the workup of anemia.

The false positive rate of FOBT is around 10%, and the false negative rate is as high as 50%. FOBTs can be thrown off by a variety of confounding factors – false positives by epistaxis, horseradish, iron supplementation (though that last point is heavily contested). False negatives can be caused by rate of bleed, volume of blood, duration of blood in upper GI tract (pancreatic enzymes degrade hemoglobin).

There are entire hospitals that have completely done away with the FOBT, which have saved significant medical expenses without adversely affecting patient outcomes. Furthermore, there is data that suggests when working up a patient for GI bleed, history and physical with a visual examination of the stool is sufficient, and that FOBT does not change management. In an era with a variety of advanced imaging modality, the FOBT is an outdated test with little clinical significance.




Mathews, B., Ratcliffe, T., Sehgal, R., Abraham, J., Monash, B. Fecal occult blood testing in hospitalized patients with upper gastrointestinal bleeding. J Hosp Med2017;12:567–569


Eliminating In-Hospital Fecal Occult Blood Testing: Our Experience with Disinvestment Gupta, Arjun et al. The American Journal of Medicine , Volume 131 , Issue 7 , 760 – 763