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Stool guaiac test

Definition

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:

  • You will need to collect a stool sample from three bowel movements, one right after the other.
  • You smear a small amount of the stool on a card for each bowel movement
  • You mail the card to a laboratory for testing.
  • A drop or two of testing solution is added to the sample at the lab. A color change is a sign of blood in the stool.

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. 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:

  • Red meat
  • Cantaloupe
  • Uncooked broccoli
  • Turnip
  • Radish
  • Horseradish

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:

  • 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 say you have blood in the stool or black, tarry stools

Normal Values

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:

  • Nosebleed
  • Coughing up blood and then swallowing it.

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

What the risks are

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

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

References

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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