248 The sensitivity of the Hemoccult test varies both with the concentration of Hb present in stool and with the extent of Hb exposure to the proteolytic effects of the digestive tract. Normal GI blood loss is limited to less than 2.5 mL/day, which translates to less than 2 mg of Hb per gram of stool (0.2% by weight). Routine iron supplementation does cause black stool but does not cause a positive Hemoccult test result despite early in vitro studies to the contrary. A positive test should be considered evidence of the presence of blood until proved otherwise. 244,245 If in doubt, the practitioner should determine the effects of povidone-iodine on the product used in his or her institution prior to performing the test on the patient's stool. 243 False-positive tests resulting from exposure to povidone-iodine were reported in the 1970s and 1980s, but this substance is not usually a potential contaminant in a rectal exam. 242 A simple in vivo study convincingly called into question the possibility of peroxidase passing through the stomach without being denatured.
241,242 Most vegetables contain peroxidase, including (in decreasing order) broccoli, turnips, cantaloupe, red radishes, horseradish, cauliflower, parsnips, Jerusalem artichokes, bean sprouts, beans, lemon rind, mushrooms, parsley, and zucchini.
False-positive results have been attributed to the ingestion of partly cooked or large quantities of meat (dietary sources of myoglobin and Hb) and peroxidase-rich food. 239–241 Slow bleeding in the upper GI tract, during which heme can be converted (denatured) to porphyrin during transit through the gut, may not be identified by stool testing. 44 Low pH, heat, dry stools, reducing substances (including antioxidants such as vitamin C), and antacids can cause false-negative findings. Testing for occult blood in stool is associated with false-positive and false-negative results, but in its primary role in emergency medical practice, the test is usually reliable in detecting significant acute gastrointestinal (GI) hemorrhage ( Fig. The addition of hydrogen peroxide developer solution will oxidize the dye to a blue color in the presence of a peroxidase (e.g., Hb). The original test used guaiac, but current tests use more sensitive and more reliable quinolone compounds. The developer contains 5% hydrogen peroxide and 75% alcohol. The test card is impregnated with a dye that exhibits a blue color reaction when oxidized. Roberts MD, FACEP, FAAEM, FACMT, in Roberts and Hedges’ Clinical Procedures in Emergency Medicine and Acute Care, 2019 Detection of Blood in Stoolīedside fecal blood tests make use of the peroxidase-like activity of Hb.
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