Guiaiac fecal occult blood test (FOBT)

The guaiac fecal occult blood test can detect small amounts of blood in stool and is more specific to finding blood from further up the digestive tract, such as the stomach.

woman sitting at laptop


The FOBT can detect small amounts of blood in the stool, which can indicate cancerous growths.

This option:

  • Detects blood in stool
  • Requires you to avoid certain red meat and other foods and medicines
  • Is low risk
  • Is generally covered by insurance

The details

The guaiac fecal occult blood test (FOBT) is used to find occult blood (or blood that can’t be seen with the naked eye) in stool.

The idea behind this test is that blood vessels at the surface of larger polyps or cancers are often fragile and easily damaged by passing stool. The damaged blood vessels usually release a small amount of blood into the stool, but only rarely is there enough bleeding to be visible in the stool. The FOBT is an easy way to determine whether there is blood in your stool, which could be the result of polyps or colorectal cancer.

The FOBT detects blood in the stool through a chemical reaction. However, it can’t tell if the blood is from the colon, rectum, or from other parts of the digestive tract, like the stomach. If this test is positive, a colonoscopy will be needed to find the reason for the presence of blood. Although cancers and polyps can cause blood in the stool, there are other causes too. Ulcers, hemorrhoids, diverticulosis (tiny pouches that form at weak spots in the colon wall) or inflammatory bowel disease (colitis) may also cause blood in your stool.

This screening test is done with a kit that you can use in the privacy of your own home. Another important part of this kit and test is that it requires you to check more than one stool sample. Also, unlike some other screening tests (including colonoscopy), this one must be repeated every year.

woman at table

Pros & cons


  • Inexpensive; covered by most insurance
  • Can be completed in the comfort of your own home


  • Cannot identify polyps; can only detect signs of cancer
  • Will need a colonoscopy if test is positive
  • Restricted diet and multiple days of stool collection
  • High false positive rate
  • May miss tumors that bleed in small amounts or not at all
man with bag in grocery store

The prep

Some foods or drugs can affect the outcome of the FOBT, so your doctor may suggest that you avoid the following before this test:

  • Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen (Advil), naproxen (Aleve) or aspirin (more than one adult aspirin per day), for seven days before testing
  • Vitamin C in excess of 250 mg daily from either supplements or citrus fruits and juices for three days before testing
  • Red meats for three days before testing, as the components of blood in the meat may cause the test to show positive

The steps

Begin with all of your supplies ready and in one place. Supplies will include a test kit, test cards, either a brush or wooden applicator and a mailing envelope. The kit will give you detailed instructions on how to collect the stool.

  1. First, you’ll need to collect a sample from your bowel movement. You can place a sheet of plastic wrap or paper loosely across the toilet bowl to catch the stool or you can use a dry container to collect the stool – whichever you prefer.
  2. Use a wooden applicator or a brush to smear a thin film of the stool sample onto one of the slots in the test card or slide.
  3. Next, collect a specimen from a different area of the same stool and smear a thin film of the sample onto the other slot in the test card or slide.
  4. Close the slots and put your name and the date on the test kit.
  5. Store the kit overnight in a paper envelope so it can dry.
  6. Repeat the test on your next two bowel movements, if instructed.
  7. Place the test kit in the mailing pouch provided and return it to your doctor or lab as soon as possible (but within 14 days of taking the first sample).
person holding phone

If you get positive results

For the FOBT, a positive test result indicates that abnormal bleeding is occurring somewhere in the digestive tract. This blood loss could be due to a number of things besides cancer, so if the test finds blood, a colonoscopy will be needed to look for the source.

Top resources