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.
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.
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 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.
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.
A negative test result means that there is no blood in the stool.
There can be false-positive and false-negative results.
Errors are reduced when you follow instructions during collection and avoid certain foods and medicines.
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
- Crohn disease or ulcerative colitis
- Peptic ulcer
Other causes of 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, usually 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.
Why the Test is Performed
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
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.
US Preventive Services Task Force, Bibbins-Domingo K, Grossman DC, et al. Screening for colorectal cancer: US Preventive Services Task Force recommendation statement. JAMA. 2016;315(23):2564-2575. PMID: 27304597