Medical Tests

Fecal Occult Blood Tests (FOBT) 

What is a fecal occult blood test? 

A fecal occult blood test (FOBT) looks at a sample of your stool (feces) to check for blood. Occult blood means that you can’t see it with the naked eye. Blood in the stool means there is likely some kind of bleeding in the digestive tract. It may be caused by a variety of conditions, including: 

  • Polyps 
  • Hemorrhoids 
  • Diverticulosis 
  • Ulcers 
  • Colitis, a type of inflammatory bowel disease 

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 women. A fecal occult blood test is a screening test that may help find colorectal cancer early when treatment is most effective. 

Other names for this test include FOBT, stool occult blood, occult blood test, Hemoccult test, guaiac smear test, gFOBT, immunochemical FOBT, iFOBT, and FIT. 

What is it used for? 

A fecal occult blood test is used as an early screening test for colorectal cancer. It may also be used to diagnose other conditions that cause bleeding in the digestive tract. 

Why do I need a fecal occult blood test? 

The National Cancer Institute recommends that people get regular screenings for colorectal cancer starting at age 50. The screening may be a fecal occult test or another type of screening test. These tests may include: 

  • A stool DNA test. For this test, you can use an at-home test kit to take a sample of your stool and return it to a lab. It will be checked for blood and genetic changes that may be signs of cancer. If the test is positive, you will need a colonoscopy. 
  • A colonoscopy. This is a minor surgical procedure. You will first be given a mild sedative to help you relax. Then a health care provider will use a thin tube to look inside your colon 

There are advantages and disadvantages to each type of test. Talk to your health care provider about which test is right for you. 

If your provider recommends a fecal occult blood test, you need to get it every year. A stool DNA test should be taken every 3 years, and a colonoscopy should be done every ten years. 

You may need screening more often if you have certain risk factors. These include: 

  • A family history of colorectal cancer 
  • Cigarette smoking 
  • Obesity 
  • Excessive alcohol use 

What happens during a fecal occult blood test? 

A fecal occult blood test is a noninvasive test that you can perform at home at your convenience. Your health care provider will give you a kit that includes instructions on how to do the test. There are two main types of fecal occult blood tests: the guaiac smear method (gFOBT) and the immunochemical method (iFOBT or FIT).  

Below are typical instructions for each test. Your instructions may vary slightly depending on the manufacturer of the test kit. 

For a guaiac smear test (gFOBT), you will most likely need to: 

  • Collect samples from three separate bowel movements. 
  • For each sample, collect the stool and store it in a clean container. Make sure the sample does not mix in with urine or water from the toilet. 
  • Use the applicator from your test kit to smear some of the stool on the test card or slide, also included in your kit. 
  • Label and seal all your samples as directed. 
  • Mail the samples to your health care provider or lab. 

For a fecal immunochemical test (FIT), you will most likely need to: 

  • Collect samples from two or three bowel movements. 
  • Collect the sample from the toilet using the special brush or other devices that was included in your kit. 
  • For each sample, use the brush or device to take the sample from the surface of the stool. 
  • Brush the sample onto a test card. 
  • Label and seal all your samples as directed. 
  • Mail the samples to your health care provider or lab. 

Be sure to follow all the instructions provided in your kit and talk to your health care provider or Care Team if you have any questions. 

Will I need to do anything to prepare for the test? 

Certain foods and drugs may affect the results of a guaiac smear method (gFOBT) test. Your health care provider may ask you to avoid the following: 

  • Nonsteroidal, anti-inflammatory drugs (NSAIDs) such as ibuprofen, naproxen, or aspirin for seven days before your test. If you take aspirin for heart problems, talk to your health care provider before stopping your medicine.  
  • Acetaminophen may be safe to use during this time but check with your health care provider or Care Team before taking it. 
  • More than 250 mg of vitamin C daily from supplements, fruit juices, or fruit for seven days before your test. Vitamin C can affect the chemicals in the test and cause a negative result even if there is blood present. 
  • Red meat, such as beef, lamb, and pork, for three days before the test. Traces of blood in these meats may cause a false-positive result. 

There are no special preparations or dietary restrictions for a fecal immunochemical test (FIT). 

Are there any risks to the test? 

There is no known risk to having a fecal occult blood test. 

What do the results mean? 

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. 

Is there anything else I need to know about a fecal occult blood test? 

Regular colorectal cancer screenings, such as the fecal occult blood test, are a valuable tool in the fight against cancer. Studies show that screening tests can help find cancer early and may reduce deaths from the disease. 

Source: MedlinePlus, National Library of Medicine