[Bacterial hemoglobin degradation in feces--the cause of false-negative occult blood tests. Results of an in vitro study].

Possible causes of false negative tests for occult fecal bleeding are hemoglobin concentrations too low to be detected or a bacterial degradation of fecal hemoglobin. In the present in vitro study the degradation of hemoglobin was assessed in cultures of fecal bacteria or in homogenates of human feces. At a hemoglobin concentration of 1.5 mg per gram stool a 48 h incubation caused a decrease of test positivity for hemoglobin of 88% (aerobic conditions) or 52% (anaerobic conditions). Thus, bacterial degradation is one of the causes of false negative test results when testing for occult fecal bleeding. These findings should be considered by the clinician when the efficiency of currently available occult blood tests in detecting right sided colonic lesions is to be assessed.