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).
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
- Uncooked broccoli
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 health care provider if you need to stop taking these before the test. Never stop or change your medicine without first talking to your 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.
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:
- Colon cancer or other gastrointestinal (GI) tumors
- Colon polyps
- Bleeding veins in the esophagus or stomach (esophageal varices and portal hypertensive gastropathy)
- Inflammation of the esophagus (esophagitis)
- Inflammation of the stomach (gastritis) from GI infections
- Inflammatory bowel disease
- Peptic ulcer
Other causes of positive test may include:
- 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.
There can be false-positive and false-negative results.
Errors are reduced when you follow instructions during collection and avoid certain foods and medicines.
Guaiac smear test; Fecal occult blood test -- guaiac smear; Stool occult blood test -- guaiac smear
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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. Philadelphia, 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. 10th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 20.
Update Date 2/4/2015
Updated by: Subodh K. Lal, MD, Gastroenterologist with Gastrointestinal Specialists of Georgia, Austell, GA. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.