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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, you use a test kit. Follow the kit instructions exactly. This ensures accurate results. In brief:

  • You collect a stool sample from three different bowel movements.
  • For each bowel movement, you smear a small amount of the stool on a card provided in the kit.
  • You mail the card to a laboratory for testing.

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.

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 Results

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

What Abnormal Results Mean

Abnormal results may be due to problems that cause 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.

Risks

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

National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines): Colorectal cancer screening. Version 2.2013. Accessed October 24, 2013.

Salwen MJ, Siddiqi HA, Gress FG, Bowne WB. Laboratory diagnosis of gastrointestinal and pancreatic disorders. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Phildelphia, Pa: Elsevier Saunders; 2011:chap 22.

Savides TJ, Jensen DM. Gastrointestinal bleeding. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran's Gastrointestinal and Liver Disease Pathophysiology/Diagnosis/Management. 9th ed. Philadelphia, Pa: Elsevier Saunders; 2010:chap 19.

Updated: 10/14/2013

George F. Longstreth, MD, Department of Gastroenterology, Kaiser Permanente Medical Care Program, San Diego, California. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.


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