Errors in the Guaiac Test For Occult Blood

To the Editor:— During a lecture and clinical demonstration, I performed a guaiac test for occult blood on a stool specimen that appeared tarry, and obtained a negative reaction. In order to illustrate how a positive reaction should appear, I repeated the test with a specimen containing fresh blood. To my discomfiture, this too, produced a negative result. This communication is written to call attention to some of the causes of such false reactions. As is well known, the guaiac test is usually performed by smearing a small quantity of stool on a piece of filter paper. Two drops of each of the following reagents are added in succession and mixed with the stool: glacial acetic acid, saturated alcoholic solution of gum guaiac, and 3% hydrogen peroxide. The presence of occult blood is indicated by the appearance of a deep blue color within one minute.¹ What is less known,