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).
Guaiac is a substance from a plant that is used to coat the FOBT test cards.
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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 3 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.
Be sure to keep 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. Follow instructions about not eating certain foods before the test. These may include:
- Red meat
- Uncooked broccoli
Some medicines may interfere with the test. These include vitamin C, aspirin, and NSAIDs such as ibuprofen and naproxen. Ask your 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.
Why the Test is Performed
This test detects blood in the digestive tract. It may be done if:
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 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 gastrointestinal infections
- Crohn disease or ulcerative colitis
- Peptic ulcer
Other causes of a positive test may include:
- Coughing up blood and then swallowing it
If the stool guaiac results come back positive for blood in the stool, your doctor will likely order other tests, often including a colonoscopy.
The stool guaiac test does not diagnose cancer. Screening tests such as colonoscopy can help detect cancer. The stool guaiac test and other screenings can catch colon cancer early, when it is easier to treat.
There can be false-positive and false-negative results.
Errors are reduced when you follow instructions during collection and avoid certain foods and medicines.
Rex DK, Boland CR, Dominitz JA, et al. Colorectal cancer screening: recommendations for physicians and patients from the US Multi-Society Task Force on colorectal cancer. Am J Gastroenterol. 2017;112(7):1016-1030. PMID: 28555630 pubmed.ncbi.nlm.nih.gov/28555630/.
Savides TJ, Jensen DM. Gastrointestinal bleeding. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran's Gastrointestinal and Liver Disease. 11th ed. Philadelphia, PA: Elsevier; 2021:chap 20.
US Preventive Services Task Force website. Final recommendation statement. Colorectal cancer: screening. www.uspreventiveservicestaskforce.org/uspstf/recommendation/colorectal-cancer-screening. Published May 18, 2021. Accessed May 27, 2022.