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.

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Overview

The guaiac test: 

  • Detects blood in stool via a chemical reaction
  • Requires three separate bowel movements for testing
  •  Requires you to avoid certain foods before and during the testing period 
  • Is available over the counter (does not require a prescription)

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 a FOBT 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.

Guaiac tests are available over-the-counter, but you should talk to your doctor first to make sure you use the type of screening that is best for you.

How to talk to your doctor about screening

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Guaiac test pros

  • Easy to use
  • Done in privacy of your home
  • Is available over the counter (does not require a prescription) 
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Guaiac test cons

  • Requires multiple bowel movements
  • Make take multiple days to complete
  • Does not provide a visual exam of the colon and rectum
  • May miss polyps or tumors that bleed in small amount or not at all
  • Must be repeated every year
  • Follow-up colonoscopy after a positive test may not be covered by insurance
  • Has dietary restrictions before and during the testing period: red meat and certain vegetables (e.g., radishes, turnips, cauliflower, cucumbers) can cause a false-positive result
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Prepping for a FOBT

Some foods or drugs can affect the outcome of the FOBT, so your doctor may ask 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 meat (beef and lamb) including processed meat and liver
  • Any RAW fruits and vegetables (especially melons, radishes, turnips and horseradish)

The FOBT test: step by step

Read any included instructions carefully once you receive your test. The typical steps are described below:

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 directions 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.
  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).
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If you get positive results

A positive FOBT test result can indicate 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. Let your doctor know right away if you have a positive result. 

How do I know which screening test is right for me?

There are many screening test options for colon cancer. Ask your doctor about your risks such as lifestyle, ethnicity, and family history, as these can help determine the best method of screening for you. 

Not sure when to begin screening? Take our brief quiz to find out!

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While screening is the most important step you can take to prevent colorectal cancer, it’s not the only one.

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