Until the middle of the 1980s, there was no commonly accepted method to test for blood in gastric contents. Methods I have personally seen used in different laboratories include urine dipsticks, orthotolidine tablets, and guaiac-impregnated filter paper. Studies have indicated that small numbers of red blood cells (RBCs) are present in most gastric aspirates without clinical evidence of bleeding. These RBCs are often sufficient to produce a reaction with orthotolidine or the urine dipsticks. The guaiac-impregnated filter paper tests used for stool occult blood are less sensitive and also appear to detect clinically significant amounts of blood but were shown to lose sensitivity at pH levels below 3.0 (literature range, 2.5-4.0). In addition, cimetadine, a medication frequently used to decrease gastric acid production, can produce false negative guaiac tests. A new guaiac test introduced in 1985 called Gastroccult is buffered so as to maintain sensitivity down to pH 1.0. In addition, the reagent contains a substance that inhibits plant peroxidase and thereby decreases chances of a false positive result from that source. Also, Gastroccult is not affected by cimetadine. Gastroccult has a sensitivity of approximately 5 mg of hemoglobin/100 ml of gastric contents (equivalent to about 50 µl of whole blood/100 ml of gastric contents), with a detection range in the literature of 30-200 µl of blood/100 ml of diluent. Although there is no consensus regarding the exact amount of gastric blood considered significant, 50 µl/100 ml seems to be acceptable. Ascorbic acid (vitamin C) inhibits both the standard guaiac test and Gastroccult. Large amounts of antacids may also inhibit the test reaction; the manufacturer states that this possibility must be taken into consideration if testing is done within 1 hour after administering the antacid.