Modification of the Hemoccult slide test for occult blood in gastric juice

The Hemoccult slide test is used frequently to test for the presence of occult blood in samples of gastric juice obtained from critically ill patients. The purpose of this study was to investigate the accuracy of this test to determine the presence of blood in human gastric juice at various pH values. Gastric aspirates were collected from 10 patients who had nasogastric tubes and were receiving nothing by mouth. These aspirates were adjusted to a range of pH values varying from 1 to 8. The aspirates were then tested by the Hemoccult slide method. The tests were repeated after (1) addition of whole blood and (2) subsequent addition of antacid. False negative results were found when the pH was below 2 and false positive results were found when the pH was between 2 to 4. When 3 drops of aspirate were added to 1 ml of a 0.2 M borate buffer at pH 8.6, and the Hemoccult slide test was performed on the resulting mixture, false negative and false positive reactions were eliminated. We conclude that the addition of the gastric juice sample to a borate buffer before performing the Hemoccult slide test makes this a reliable method of determining the presence or absence of occult blood.