False positive and negative hemoccult reactions on a normal diet and effect of diet restriction.

The reliability of Hemoccult, a guaiac-impregnated slide for detecting faecal occult blood, was assessed on both normal and restricted diets. Two-hundred-and-twenty-five faecal samples were obtained from 40 patients on a normal diet, and 115 samples from 20 patients on a restricted diet. A single Hemoccult I test was performed on each sample. Faecal blood was quantitated by 51Cr red cell labelling. On a normal diet, the false positive rate was 11%. This reduced to 2% on a high fibre diet free of red meat and raw fruit and vegetables. This false positive rate is lower than has been previously reported for other, more widely-used, occult blood tests. The overall false negative rate was 48%; this diminished with increasing faecal blood loss.