A fecal occult blood (FOBT) test finds blood in the stool by placing a small sample of stool on a chemically treated card, pad, or cloth wipe. Then a special chemical solution is put on top of the sample. If the card, pad, or cloth turns blue, there is blood in the stool sample.

An FOBT may be done to check for some intestinal conditions or colorectal cancer. Colorectal cancer affects the large intestine (colon) and the rectum. Blood in the stool may be the only symptom of colorectal cancer, but not all blood in the stool is caused by cancer. Other conditions that can cause blood in the stool include:

• Hemorrhoids. These are enlarged, swollen veins in the anus. Hemorrhoids can develop inside the anus (internal hemorrhoids) or outside of the anus (external hemorrhoids).
• Anal fissures. These are thin tears in the tissue that lines the anus (anal sphincters) up into the anal canal.
• Colon polyps. These growths of tissue often look like a stem or stalk with a round top that is attached to the colon.
• Peptic ulcers. These craterlike sores develop when the digestive juices made in the stomach eat away the lining of the digestive tract.
• Ulcerative colitis. This type of inflammatory bowel disease (IBD) causes inflammation and craterlike sores (ulcers) in the inner lining of the colon and rectum.
• Gastroesophageal reflux disease (GERD). This is the abnormal backflow (reflux) of food, stomach acid, and other digestive juices into the esophagus.
• Crohn's disease. This type of inflammatory bowel disease causes inflammation and ulcers that may affect the deep layers of the lining of the digestive tract.
• Use of aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs).

A fecal occult blood test may be used to check for colorectal cancer, but it is never used to diagnose this condition. Other tests for colorectal cancer include a digital rectal examination, flexible sigmoidoscopy, colonoscopy, or CT scan (virtual colonoscopy).

Checking for hidden (occult) blood in the stool can be done at home. You can buy a test kit at a pharmacy without a prescription, or your health professional can order a test kit for you to use at home. If a home fecal occult blood test finds blood in your stool, call your health professional.

For more information on tests for colorectal cancer, see:

Which test should I have to screen for colorectal cancer?

Health Tools

Health Tools help you make wise health decisions or take action to improve your health.

Decision Points focus on key medical care decisions that are important to many health problems.