The fecal occult blood test (FOBT) is a lab test used to check stool samples for hidden (occult) blood.
Occult blood in the stool may indicate colon cancer or polyps in the colon or rectum — though not all cancers or polyps bleed.
Typically, occult blood is passed in such small amounts that it can be detected only through the chemicals used in a fecal occult blood test.
If blood is detected through a fecal occult blood test, additional tests may be needed to determine the source of the bleeding. The fecal occult blood test can only detect the presence or absence of blood — it doesn’t indicate potential sources of bleeding.
Blood in the stool may also be a sign of colorectal cancer, a type of cancer that starts in the colon or rectum. Colorectal cancer is the second leading cause of cancer-related deaths in the United States and the third most common cancer in men and in women. A fecal occult blood test is a screening test that may help find colorectal cancer early, when treatment is most effective.
Other names: FOBT, stool occult blood, occult blood test, Hemoccult test, guaiac smear test, gFOBT, immunochemical FOBT, iFOBT; FIT.
What Causes Blood to Appear in Stool?
Blood may appear in the stool because of one or more of the following conditions:
- Benign (non-cancerous) or malignant (cancerous) growths or polyps of the colon
- Hemorrhoids (swollen blood vessels near the anus and lower rectum that can rupture, causing bleeding)
- Anal fissures (splits or cracks in the lining of the anal opening)
- Intestinal infections that cause inflammation
- Ulcerative colitis
- Crohn’s disease
- Diverticular disease, caused by outpouchings of the colon wall
- Abnormalities of the blood vessels in the large intestine
- Gastrointestinal bleeding may be microscopic (invisible to the eye) or may be easily seen as red blood or black tar-like bowel movements, called melena.
Purpose of Test
Your doctor may order a fecal occult blood test to determine if you might have an underlying condition such as colon polyps, diverticulosis, hemorrhoids, an ulcer, an inflammatory bowel disease called colitis, or colorectal cancer.3 Each of these (and others) can cause bleeding in your digestive tract that ends up in your stool.
This test is not diagnostic, but would serve as an impetus for additional testing.1
While a fecal occult blood test is used to look for the presence of blood in your feces, the test doesn’t show where the bleeding, if present, is coming from or what’s causing it.
Your doctor may order an FOBT for these reasons:
Screening for Colorectal Cancer
The FOBT is most commonly used as a screening tool to look for blood in the stool of people who are 50 years or older who are at average risk of developing colorectal cancer.4 Doctors usually recommend that people who meet these criteria have an FOBT done yearly.
Colorectal cancer is the number two cause of cancer-related deaths in the United States, as well as the third most common type of cancer, and it often doesn’t have any obvious symptoms until it’s in the later stages.5
Most cases start with benign, or harmless, colon polyps, which are growths of tissue that are actually quite common in people over the age of 50 years. However, some of these polyps become precancerous or cancerous, causing them to grow and bleed in microscopic amounts when stool passes by.6 This bleeding is considered an early sign of colorectal cancer and is sometimes the only early sign, which is why regular screening for blood in your stool is so crucial. Catching cancer early can greatly improve your treatment outcome.7
One potential drawback of the FOBT is that it doesn’t always accurately show what’s going on in your body. For instance, some cancer and colon polyps don’t bleed, which could lead to a false-negative test result when you actually do have cancer or polyps.8 Conversely, you may get a false-positive result due to factors such as not adhering to the dietary and medication restrictions your doctor gave you before and during the test, or because you have a minor gastrointestinal infection, hemorrhoids, or another condition, rather than polyps or cancer.
Looking for Possible Causes of Anemia
Your doctor may order an FOBT if you have signs and symptoms of anemia, a condition in which your body doesn’t have enough red blood cells to carry oxygen through your bloodstream effectively.9 Common symptoms include fatigue, pale skin, low hemoglobin count in your blood, and weakness.
If you’ve already been diagnosed with anemia, your doctor may also recommend an FOBT if the cause of your anemia hasn’t been found. The test results can show if blood loss from your digestive tract may be part of the cause of your anemia and give your doctor direction on how to proceed.10
For instance, if your test is positive for blood in your stool, your doctor knows to then test you further for conditions that could cause the bleeding, such as ulcers, diverticulosis, or colon polyps. If your test shows that there isn’t any blood in your stool, your doctor will likely move on to look for another potential cause of your anemia.
How you prepare
The fecal occult blood test results are largely affected by how you prepare for the test, so it is important to follow the instructions carefully.
Various foods, dietary supplements and medications can affect the results of some fecal occult blood tests — either indicating that blood is present when it isn’t (false-positive) or missing the presence of blood that’s actually there (false-negative). Your doctor may ask you to avoid certain foods or medicines. To ensure accurate test results, follow your doctor’s instructions carefully.
For about three days before the test, your doctor may ask you to avoid:
- Certain fruits and vegetables, including broccoli and turnips
- Red meat
- Vitamin C supplements
- Pain relievers, such as aspirin and ibuprofen (Advil, Motrin IB, others)
What you can expect
There are several types of fecal occult blood tests, each with a different approach to collecting and testing stool. They include:
- Guaiac fecal occult blood test (gFOBT). Your doctor typically gives you a test card with room for two or three samples or two or three test cards.You collect a stool sample from each of two or three bowel movements in a clean container, usually taken on consecutive days, and then use an applicator stick to apply a smear of stool to a specific area of a card.After the samples are dry, you return them to your doctor or a designated lab, by mail or in person.
- Flushable reagent pad or tissue. You can get this kit at a store without a prescription.You place the pad or tissue in the toilet bowl after a bowel movement, usually on three consecutive days. The pads change color when blood is present.You then report the changes to your doctor, usually on a mail-in form.
- Immunochemical fecal occult blood test (iFOBT, or FIT). The collection method for this test may depend on the manufacturer, but typically, you use a special spoon or other device to collect a sample of stool and store it in a collection container that comes with the test kit.The collection container is then returned to your doctor or a designated lab, by mail or in person.Immunochemical testing is newer than gFOBT. It doesn’t require any dietary restrictions before sample collection, and testing can often be performed on a random stool sample. Immunochemical testing is also more sensitive than is gFOBT.
For accurate results, follow the instructions and return the samples promptly.
Your doctor will review the results of the fecal occult blood test and then share the results with you.
Negative result. A fecal occult blood test is considered negative if no blood is detected in your stool samples. If you had the test to screen for colon cancer and you’re at average risk — you have no colon cancer risk factors other than age — your doctor may recommend waiting one year and then repeating the test.
Positive result. A fecal occult blood test is considered positive if blood is detected in your stool samples. You may need additional testing — such as a colonoscopy — to locate the source of the bleeding.
If your results are positive for either type of fecal occult blood test, it means you likely have bleeding somewhere in your digestive tract. But it does not necessarily mean you have cancer. Other conditions that may produce a positive result on a fecal occult blood test include ulcers, hemorrhoids, polyps, and benign tumors. If your test results are positive for blood, your health care provider will likely recommend additional testing, such as a colonoscopy, to figure out the exact location and cause of your bleeding. If you have questions about your results, talk to your health care provider.