Stool guaiac test


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).

Alternative Names

Guaiac smear test; Fecal occult blood test - guaiac smear; Stool occult blood test - guaiac smear

How the Test is Performed

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:

There are many ways to collect the samples:

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.

Always follow the manufacturer's instructions on how to collect the stool. This helps the test be more accurate.

How to Prepare for the Test

Some foods can affect test results. Do not eat the following foods for 3 days before the test:

Some medicines may interfere with the test. These include vitamin C, aspirin, and nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and naproxen. Ask your doctor or nurse if you need to stop taking these before the test. Never stop or change your medicine without first talking to your health care provider.

How the Test Will Feel

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.

Why the Test is Performed

This test detects blood in the digestive tract. It may be done if:

Normal Results

A negative test result means that there is no blood in the stool.

What Abnormal Results Mean

Abnormal results may be caused by anything that causes bleeding in the stomach or intestinal tract, including:

Other causes of  positive test may include:

Abnormal tests require follow-up with your doctor. In many cases, no explanation for the abnormal result is found.


There can be false-positive and false-negative results.

Errors are reduced when you follow instructions during collection and avoid certain foods and medicines.


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.

Blanke CD, Faigel DO. Neoplasms of the small and large intestine. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 199.

Review Date: 11/5/2012
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, David R. Eltz, Stephanie Slon, and Nissi Wang.
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