The use of the Haemoccult test for detection of blood in gastric aspirates.

The guaiac test has recently been used as a determinant of gastric mucosal bleeding in trials comparing different therapeutic regimens for the prevention of gastric stress ulceration. A prospective study, evaluating the efficacy of cimetidine in controlling the gastric pH in 44 patients in our intensive care unit, demonstrated a significant direct correlation between the pH of the gastric aspirate and the Haemoccult test result. The percentage of positive test results decreased progressively with declining pH. Since this seemed to contradict all modern concepts of gastric mucosal damage, we investigated further the pH-dependent sensitivity in vitro. Using different test solutions (NaCl, gastric juice, duodenal juice, and bile) we demonstrated a strong pH-dependent decrease of sensitivity to detect blood below a pH of 4. These results led to the conclusion that the guaiac test is not a valid measure of gastric mucosal bleeding unless used on buffered gastric aspirates.