Occult (hidden or unseen) GI bleeding is not visible to the patient or physician. It usually presents as iron deficiency anemia or is identified by testing the stool for occult blood.

Red-colored stools resembling hematochezia may be caused by foods with red dyes (eg, Kool-Aid or fruit punch), beets, red licorice, or rifampin.

Black-colored stools resembling melena may be caused by ingestion of bismuth, activated charcoal, or iron, and occasionally by consumption of large quantities of chocolate, blueberries, or dark green foods. Testing is particularly important in children with equivocal symptoms, such as a well-appearing child with small amounts of red material in the stool.

The most common assays can be performed at the bedside and are guaiac-based. This test identifies hemoglobin by the presence of a peroxidase reaction that turns guaiac-impregnated paper blue [50]. Occasionally, false-positive results may occur if the patient has ingested rare red meat or peroxidase-containing vegetables (such as turnips, horseradish, broccoli, cauliflower, and cantaloupe). Whether iron supplements cause false-positive results is controversial [9]. False negative results can be obtained if the patient is ingesting large doses of ascorbic acid (vitamin C) or if intestinal bacteria have degraded the hemoglobin to porphyrin.

Patients with positive guaiac tests that are thought to be possible false-positives can be further evaluated with non-guaiac-based tests, which are based on measurement of fecal hemoglobin or fecal alpha-1 antitrypsin. The sensitivity of the test for fecal hemoglobin is limited because of degradation of hemoglobin by fecal bacteria, and the overall sensitivity and specificity are similar to the guaiac test [9]. Fecal alpha-1 antitrypsin is used primarily to diagnose enteric protein loss, but it is also elevated in patients with gastrointestinal blood loss (upper or lower), in whom it has a sensitivity and specificity of 88 and 90 percent, compared with 68 and 73 percent, respectively, of the guaiac test [9]. The fecal alpha-1 antitrypsin test can be used in patients with recurrent red stools that are guaiac-negative or in those with a possible false-positive guaiac test result.

 

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