Stool guaiac test

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.

How the Test is Performed

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:

  • You will need to collect a stool sample from three bowel movements, one right after the other.
  • You will then smear a small amount of stool on a card, and mail it to a laboratory for testing.

There are many ways to collect the samples:

  • You can catch the stool on plastic wrap that is loosely placed over the toilet bowl and held in place by the toilet seat. Then put the sample in a clean container.
  • Some test kits supply a special toilet tissue that you use to collect the sample, then put the sample in a clean container.

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.

How to Prepare for the Test

Do not eat red meat, any blood-containing food, 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 and nonsteroidal anti-inflammatory medicines (NSAIDs) such as ibuprofen and aspirin.

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.

How the Test Will Feel

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.

Why the Test is Performed

This test detects blood in the digestive tract for the following reasons:

  • You are being screened or tested for colon cancer
  • You have abdominal pain, changes in bowel movements, or weight loss
  • You have anemia (low blood count)
  • You report blood in the stool or black, tarry stools

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:

Stool guaiac testing is sometimes used to screen for colon cancer. By itself, it is not a reliable test for this purpose.

Other, non-GI-related causes of positive guaiac test may include:

  • Nosebleed
  • Coughing up blood

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.

Alternative Names

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


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.

Updated: 4/12/2012

Reviewed by: Todd Eisner, MD, Private practice specializing in Gastroenterology, Boca Raton, FL. Clinical Instructor, Florida Atlantic University School of Medicine. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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