Fecal Occult Blood Test (FOBT)
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:
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Why It Is Done
A fecal occult blood test (FOBT) is done:
- To find the presence of blood in the stool. Blood in the stool may be caused by hemorrhoids, anal fissure, colon polyps, colorectal cancer, and many other conditions that cause bleeding in the gastrointestinal tract.
- As a screening test for cancer of the colon and rectum (colorectal cancer). FOBT is a useful tool to screen for colorectal cancer because tissue or polyps with cancer changes are more likely to bleed than normal colon tissue. Polyps and cancers generally grow slowly and they may not bleed all the time. Sometimes blood in the stool is the only symptom of colorectal cancer. An FOBT helps find blood in the stool. More tests will need to be done to diagnose the cause of the bleeding. It is important to call your health professional if a home test shows blood in your stool. A home fecal occult blood test does not replace the need for a regular examination by your health professional.
- To help find the cause of abdominal pain.
- To check for the cause of anemia.
- As part of a routine physical examination for those with a higher chance of developing colorectal cancer, especially at age 50 and older.
How To Prepare
Since colorectal cancers do not bleed all the time, the test for blood in the stool is done over several days on three different stool samples. This increases the chance of finding blood in your stool.
Before doing a fecal occult blood test (FOBT), avoid the following for 2 to 3 days before the test:
- Turnips, beets, radishes, horseradish, artichokes, mushrooms, broccoli, bean sprouts, cauliflower, apples, oranges, bananas, grapes, and melon. These foods can cause the test to be positive for blood when blood is not in the stool (false-positive test results).
- Red meat, which may cause false test results. Small amounts of chicken, turkey, or fish will not affect the test.
- Iron supplements
- Aspirin (or products that contain aspirin) and nonsteroidal anti-inflammatory drugs (NSAIDs) or other medicines that irritate the stomach or intestines
- Vitamin C supplements
Do not do the test during your menstrual period or if you have active bleeding from hemorrhoids. Also, do not test a stool sample that has been in contact with toilet bowl cleaning products that turn the water blue.
Talk to your health professional about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?) .
How It Is Done
There are different home tests for fecal occult blood. It is important to follow the manufacturer's instructions provided with any test. For most tests, you will use stool samples from three different bowel movements over three different days.
For any home test, follow these general guidelines:
- Check the expiration date on the package. Do not use a test kit after its expiration date. The chemicals in the kit may not work properly after that date.
- Store the test kit as instructed. Many kits need to be stored in a refrigerator or cool place.
- Read the instructions that come with your test carefully and completely before doing the test. Pay attention to any special preparations you need to take before doing the test, such as not eating certain foods or limiting your physical activity.
- Follow the instructions exactly. Do all the steps, in order, without skipping any of them.
- If a step in the test needs to be timed, use a watch. Do not guess at the timing.
- If you are color-blind or have trouble seeing colors, have someone else read the test results for you. Most test results are color changes on a test strip.
- Record the results of the test so you can discuss them with your health professional.
The following instructions are for one of the most common tests used to find blood in the stool.
Stool guaiac cards
- Complete the identification information on the front of each card.
- During a bowel movement, collect a small amount of stool on one end of an applicator. You might try catching the stool on some plastic wrap draped loosely over the toilet bowl and held in place by the toilet seat. If you use a container to collect the stool, first clean and rinse it well to get rid of any substance that may affect the test results.
- Apply a thin smear of stool inside box A.
- Reuse the same applicator to obtain a second sample from a different part of the stool. Apply a thin smear inside box B.
- Close the cover of the slide.
- Complete the remaining two cards in the same way for two other bowel movements.
- You probably will be instructed to return all slides to your health professional either in person or by mail within 4 days of collecting the samples.
- If your test kit has the developer solution, wait 3 to 5 minutes before you put 1 drop of the developer solution to the area with the stool. Put 1 drop of the developer solution to the control areas of the card so that you will know what positive and negative test results should look like. An area to read the results is found on the reverse side of the card. Turn the card over and read the results within 10 seconds.
Other test kits
- Some kits instruct you to use a special cloth to wipe with after a bowel movement. After wiping with the cloth, you put the developer solution on it to check for color change that means there is blood in the stool.
- Other kits have a special test pad that you place in the toilet after having a bowel movement. The pad will change color if the stool has blood in it.
If you find blood in your stool, call your health professional as soon as possible.
How It Feels
You may find it unpleasant to collect a stool sample for a fecal occult blood test (FOBT).
There is no chance for problems with collecting a stool sample.
A fecal occult blood test finds blood in the stool by placing a small sample of stool on a chemically treated card, pad, or 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.
You can read the results for some FOBT kits. Other tests are read by your health professional.
A normal test (no color change) means that there was no blood in your stool at the time of the test. Normal test results are called negative.
An abnormal test (blue color change) means that there was some blood in your stool at the time of the test. Abnormal test results are called positive.
If the test does not find blood in your stool, that does not mean colorectal cancer or colon polyps are not present (false-negative). FOBT is positive in 4 out of 10 people who have colorectal cancer.1 Talk with your health professional about how often you should do a test depending on your age and any risk factors you may have for colorectal cancer.
If the test finds blood in your stool, it does not always mean you have cancer (false-positive). An FOBT has a high rate of false-positive results. This can occur because the blood comes from another source, such as from red meat you have eaten, menstrual bleeding, hemorrhoids, Crohn's disease, ulcerative colitis, a stomach ulcer, the use of aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs), and many other causes.
A colon polyp, a precancerous polyp, or cancer can cause a positive test. With a positive test, there is a small chance that you have early-stage colorectal cancer.1
If blood is found in your stool, talk with your doctor about what test you may need next.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Taking some medicines, such as aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs), or anticoagulants.
- Using laxatives, vitamin C, or iron supplements.
- Having blood in the urine, menstrual bleeding, hemorrhoids, an anal fissure, bleeding gums, or nosebleeds.
- Eating certain vegetables, such as radishes, turnips or beets, or red meat cooked rare 2 days before the test.
- Using a toilet bowl cleaner at the time of the test.
What To Think About
- A fecal occult blood test (FOBT) is most often used as a screening test. By itself, an FOBT is not used to diagnose colon polyps or colorectal cancer. FOBT is a more effective screening test if it is done every year. If an FOBT finds blood in the stool, you may need more tests, such as a rectal exam, colonoscopy, barium enema, or endoscopy. For more information, see the medical tests Digital Rectal Examination (DRE), Colonoscopy, Barium Enema, Upper Gastrointestinal Endoscopy, Sigmoidoscopy (Anoscopy, Proctoscopy), and Computed Tomography Colonography (CTC).
- An FOBT has a high rate of false-positive results. This means that the test may be positive when you do not have a polyp or cancer. This can occur because the blood comes from another source, such as from hemorrhoids. Tests, such as a colonoscopy, may be done to find the cause of the positive FOBT and to rule out cancer.
- People ages 50 to 80 who have an FOBT every year are less likely to die of colorectal cancer than people who do not have regular FOBTs.2
- There are other tests where you don't need to limit what you eat prior to the test. Also, you may not need to provide as many stool samples. These tests are called the immunochemical Fecal Occult Blood Test (iFOBT) or the Fecal Immunochemical Test (FIT).
- Which screening test you choose depends on your risk, your preference, and your doctor. Talk to your health professional about your risk factors and what test is best for you.
- U.S. Preventive Services Task Force (2002). Screening for colorectal cancer. Available online: http://www.ahrq.gov/clinic/uspstf/uspscolo.htm.
- Lewis C (2007). Colorectal cancer screening, search date November 2006. Online version of BMJ Clinical Evidence. Also available online: http://www.clinicalevidence.com.
Other Works Consulted
- Helfand M (2005). Colorectal cancer section of Adult preventive health care. In DC Dale, DD Federman, eds., ACP Medicine, section 1, chap. 5. New York: WebMD.
- Levin B, et al. (2008). Screening and surveillance for the early detection of colorectal cancer and adenomatous polyps, 2008: A joint guideline from the American Cancer Society, the U.S. Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. CA: A Cancer Journal for Clinicians, 58(3): 130–160.
- Nadel MR, et al. (2005). A national survey of primary care physician's methods for screening for fecal occult blood. Annals of Internal Medicine, 142(2): 86–94.
|Editor||Kathleen M. Ariss, MS|
|Associate Editor||Tracy Landauer|
|Primary Medical Reviewer||Kathleen Romito, MD - Family Medicine|
|Specialist Medical Reviewer||Peter J. Kahrilas, MD - Gastroenterology|
|Last Updated||August 11, 2008|
Last Updated: August 11, 2008