The stool guaiac test finds hidden (occult) blood in the stool (bowel movement), even when you cannot see the blood yourself.
It is the most common form of fecal occult blood test (FOBT) in use today.
Guaiac smear test; Fecal occult blood test - guaiac smear; Stool occult blood test - guaiac smear
In the most common way of doing a stool guaiac test, a small sample of stool is placed on a paper card and a drop or two of testing solution is added. A color change is a sign of blood in the stool.
If the test is performed in an office or hospital, a doctor may collect a small amount of stool during a rectal examination.
If the test is performed at home:
There are many ways to collect the samples:
Do not take stool samples from the toilet bowl water, because 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.
To make sure the guaiac test is accurate, follow the manufacturer's instructions on how to collect the stool.
Do not eat red meat, cantaloupe, uncooked broccoli, turnip, radish, or horseradish for 3 days before the test. These foods can sometimes interfere with the test.
You may need to stop taking medicines that can interfere with the test. These include vitamin C, aspirin, and nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and naproxen.
Check with your health care provider about medication changes you may need to make. Never stop or decrease any medication without first talking to your health care provider.
There is no discomfort when the test is done at home, because this test only involves a normal bowel movement. If stool is collected during an exam, there may be some discomfort in the anal canal and rectum.
This test detects blood in the digestive tract for the following reasons:
A negative test result means that there is no blood in the stool.
Abnormal results may be caused by anything that causes bleeding in the stomach or intestinal tract, including:
Other causes of positive guaiac test may include:
Abnormal tests require follow-up with your doctor. In many cases, however, no explanation for the abnormal result is found.
There can be false-positive and false-negative results. Using the right collection technique, avoiding certain drugs, and observing food restrictions can reduce errors.
Tack J. Dyspepsia. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran's Gastrointestinal and Liver Disease. 9th ed. Philadelphia, Pa: Saunders Elsevier; 2010:chap 13.