The stool guaiac test looks for hidden (occult) blood in a stool sample. It can find blood even if you cannot see it yourself. It is the most common type of fecal occult blood test (FOBT).
Guaiac is a substance from a plant that is used to coat the FOBT test cards.
Usually, you collect a small sample of stool at home. Sometimes, a doctor may collect a small amount of stool from you during a rectal examination.
If the test is done at home, you use a test kit. Follow the kit instructions exactly. This ensures accurate results. In brief:
DO NOT take stool samples from the toilet bowl water. This can cause errors.
For infants and young children wearing diapers, you can line the diaper with plastic wrap. Place the plastic wrap so that it keeps the stool away from any urine. Mixing of urine and stool can spoil the sample.
Some foods can affect test results. Follow instructions about not eating certain foods before the test. These may include:
Some medicines may interfere with the test. These include vitamin C, aspirin, and NSAIDs such as ibuprofen and naproxen. Ask your health care provider if you need to stop taking these before the test. Never stop or change your medicine without first talking to your provider.
The at-home test involves a normal bowel movement. There is no discomfort.
You may have some discomfort if the stool is collected during a rectal exam.
This test detects blood in the digestive tract. It may be done if:
A negative test result means that there is no blood in the stool.
Abnormal results may be due to problems that cause bleeding in the stomach or intestinal tract, including:
Other causes of positive test may include:
If the stool guaiac results come back positive for blood in the stool, your doctor will likely order other tests, usually including a colonoscopy.
The stool guaiac test does not diagnose cancer. Screening tests such as colonoscopy can help detect cancer. The stool guaiac test and other screenings can catch colon cancer early, when it is easier to treat.
There can be false-positive and false-negative results.
Errors are reduced when you follow instructions during collection and avoid certain foods and medicines.
Reviewed By: Michael M. Phillips, MD, Clinical Professor of Medicine, The George Washington University School of Medicine, Washington, DC. Internal review and update on 01/19/2019 by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.