Occult Faecal Blood Loss Determined by Chemical Tests and a51Cr Method

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Abstract
Stools from volunteers participating in an acetylsalicylic acid (ASA) study were examined during periods with restrictive and liberal diets. With 51Cr-determined fecal blood less than 2 ml/100 g, the rates of positive benzidine and 3,3'',5,5''-tetramethylbenzidine (TMB) tests were about 5 and 25% for periods with restrictive and liberal diets, respectively. Similarly, Fecatest showed 2% positive tests on a restrictive and 8% on a liberal diet, whereas Hemoccult II slide and BM-test-Hemafecia showed < 5% positive tests on the 2 diets. When ASA-induced blood loss (mainly gastric) exceeded 5 ml/100 g feces, the rate of positive benzidine and TMB tests, including the Hemo-Fec Test, varied from 87 to 100%. Fecatest detected half of these cases, whereas Hemoccult II and BM-test-Hemafecia were positive in < 1/3. By repeated analyses of fecal specimens stored for 3 days, Fecatest showed a substantially increased sensitivity. Benzidine and TMB tests are apparently sensitive enough to detect occult blood loss from all levels of the gastrointestinal tract, but dietary restrictions are essential to reduce the rate of false-positive tests. Guaiac tests, perhaps with the exception of Fecatest, should be reserved for the detection of occult blood loss from the lower gastrointestinal tract.