The stool guaiac test or occult stool blood test is a generally self-administered diagnostic test for screening for colon cancer. It consists of a card with six sample areas where the patient applies a small sample of stool. The area is then sealed with an adhesive tab and sent to a laboratory. There, hydrogen peroxide is applied to the sample. A rapid change to a brilliant blue color indicates that blood can be present in the sample.
Although the test is an important early diagnostic tool, false positives are very common. Heme in meat can react in a similar manner, as can some substances in raw vegetables. Vitamin C should be avoided because it can inhibit the reaction.
A colonoscopy should follow every positive result, although even a "true positive" generally does not indicate colon cancer but some other disorder that can cause bleeding such as hemorrhoids, Celiac disease, GERD, ulcers or inflammatory bowel disease.