The guaiac fecal occult blood test (gFOBT) is used to screen for colorectal cancer. Cancer in the colon or polyps in the colon are more likely to bleed than normal colon tissue is. This test uses a chemical to check for very small amounts of blood in the stool.

If signs of blood are found in the stool sample, more tests, such as a colonoscopy, will be done to find the cause.