What Is a Fecal Occult Blood Test (FOBT)?

A Non-Invasive Way to Check for Blood in the Stool

A fecal occult blood test (FOBT), also known as a stool occult test or hemoccult test, is a non-invasive way to screen your feces, or stools, for blood that's not visible to the naked eye, known as occult blood. If there's blood in your stool, this indicates that there's likely bleeding somewhere in your digestive tract, which can be a symptom of several conditions, as well as a sign of colorectal cancer.

Common types of fecal occult blood tests are the guaiac smear test (gFOBT) and the fecal immunochemical test (FIT), also known as an immunochemical fecal occult blood test (iFOBT).

types of fecal occult blood tests

Verywell / Cindy Chung

Purpose of Test

Your doctor may order a fecal occult blood test to help determine if you might have an underlying condition such as colon polyps, diverticulosis, hemorrhoids, an ulcer, an inflammatory bowel disease such as ulcerative colitis, or colorectal cancer. Each of these (and others) can cause bleeding in your digestive tract that ends up in your stool.

This test is not diagnostic, but instead shows that additional testing is needed. 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.

Screening for Colorectal Cancer

FOBT is most commonly used to look for blood in the stool as a screening method for colorectal cancer. It's recommended that people 45 years or older who are at average risk of developing colorectal cancer have a fecal immunochemical test (FIT) done yearly or a colonoscopy at least every 10 years.

Colorectal cancer is the second most common cause of cancer-related deaths in the United States, as well as the third most common type of cancer in men and women. It often doesn't have any obvious symptoms until it's in the later stages.

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.

This bleeding can be an early sign of colorectal cancer—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.

Looking for Possible Causes of Anemia

Another reason your doctor may order an FOBT is 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. Common signs and symptoms include fatigue, pale skin, weakness, and a low hemoglobin count in your blood.

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.

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.

Risks

Because this is a non-invasive test, there are no physical risks involved with the FOBT itself.

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.

Conversely, you may get a false-positive result due to factors such as having a minor gastrointestinal infection, hemorrhoids, or another condition, rather than polyps or cancer.

Before the Test

Your doctor will prescribe or provide you with an FOBT kit, which will include instructions on how to collect your stool samples at home. Preparation for the test will depend on which method you've been asked to use:

  • Guaiac smear test (gFOBT): This test uses a chemical that changes color when blood is present. Typically, you collect three stool samples on three different days and send them to a lab for analysis. This test requires that you abstain from certain foods and medications beforehand to help ensure accuracy.
  • Fecal immunochemical test (FIT): A newer test for evaluating fecal occult blood, the FIT uses antibodies to detect blood and has largely replaced gFOBT for colorectal screening because it can be more sensitive in detecting cancer. It doesn't require any special dietary restrictions and can be done at any time. With this test, you may collect one to three stool samples as your doctor instructs and send them to a lab.

Your doctor may also recommend the purchase of a flushable reagent pad or tissue. This test is available over-the-counter without a prescription and requires you to put the pad or tissue in the toilet with your bowel movement on three separate days. It works similarly to the gFOBT by using a chemical that changes color if there's blood and, like the gFOBT, there are dietary restrictions you need to follow before you do the test. You'll read the results yourself and send them to your doctor.

Though this test is fairly sensitive, many doctors prefer to do the gFOBT or the FIT because of the chance that patients won't be able to interpret the test results as accurately as a lab technician would.

Timing

The total time this test takes will depend on instructions provided by your doctor and the method that's being used.

  • If you're having a guaiac smear test, you will dedicate about a week and a half for test preparation and sample collection. The entire process, including evaluation of sample(s) and delivery of results, may take up to four weeks.
  • With a fecal immunochemical test, you'll spend just a few minutes collecting your samples. Factoring in delivering the samples and evaluation, the whole process may take two to three weeks, start to finish. This depends on the lab, but largely has to do with the fact that you need to collect three different samples on separate days if you are screening for colorectal cancer or testing for bleeding in your digestive tract due to anemia.
  • The flushable reagent pad or tissue test doesn't take as long as the other two methods because you read and record the results yourself after three bowel movements on consecutive days, then send the results to your doctor. The total time for this method is around six to 10 days, including preparation.

Location

You collect the stool samples for these tests in your home and then bring or mail them to your doctor's office, or send them to a lab to be analyzed for the presence of blood. In the case of the reagent pad or tissue test, you perform this test and record the results at home.

Foods and Medications

To prepare for the gFOBT or the flushable reagent pad/tissue test, you will be instructed to avoid certain foods and medications for a period of time before you collect stool samples or perform the test, as applicable.

Your doctor will give you specific instructions, but you will likely need to avoid the following before and during your gFOBT or reagent pad test in order to get the most accurate results:

  • Nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin, Aleve (naproxen), or Motrin/Advil (ibuprofen), can cause bleeding in your stomach or intestines, which may cause a false-positive on your test. In this case, a false-positive means that the test will show you have blood in your stool but it's from NSAID-induced bleeding, not cancer or polyps. It's ideal to avoid NSAIDs for seven days before your FOBT and while you are taking stool samples, if possible.
  • Red meat, like beef, lamb, and pork, and other foods like broccoli, turnips, horseradish, and cauliflower, can also cause false-positives on the gFOBT. These should be avoided for three days before your test and throughout the testing process.
  • Other medications, such as Colcrys (colchicine), which is used to treat gout, oxidizing drugs like iodine supplements, often used for thyroid disease, and boric acid, which may be used to treat fungal infections, can create a false-positive result by reacting with the chemicals in the test. Your doctor may recommend that you avoid these for three days before your test or that you do a different screening method.
  • Vitamin C can affect the chemicals in the test and may create the opposite problem—they can cause a false-negative. A false-negative means the test will show you don't have blood in your stool when you actually do. Vitamin C supplements, juices with added vitamin C, or any amount of fruit that will give you more than 250 mg of vitamin C per day, should be avoided for three to seven days before your FOBT and during the testing process.

As mentioned, you won't need to avoid any food or medication if you're doing the FIT.

Cost and Health Insurance

The cost of an FOBT depends on what method is used, as well as what laboratory your results come from, but it's considered a low-cost test.

If you have health insurance, the Affordable Care Act (ACA) requires that both Medicare and private insurers cover all the costs of colorectal cancer screening tests, including FOBTs. This means that you won't have to pay anything out of your own pocket if you have an FOBT to screen for colorectal cancer.

However, if you're on a health insurance plan that was grandfathered in, meaning that it existed before 2010 when the ACA was enacted, these requirements don't apply to your insurer. This doesn't mean that your FOBT won't be covered, but you should contact your insurance company or someone in human resources at your place of work to find out for sure.

If you're having an FOBT to look for causes of anemia and you have insurance, it will likely be covered as any necessary test would be, though you may still have to pay a deductible and/or coinsurance, depending on your plan. Contact your health insurance company at the number on the back of your insurance card if you have any questions about this.

If you're on Medicaid, coverage for this test as a colorectal cancer screening will depend on what state you're in. Contact your local Medicaid office to find out more.

During the Test

Be sure to follow the instructions provided by your doctor and that accompany your kit exactly, as there can be variations between manufacturers.

Pre-Test

Remember that you will need to make sure that you have followed the specific dietary restrictions your doctor gave you, if any, before proceeding to take samples of your stool. If you don't, your results may not be accurate.

Throughout the Test

Once you're ready to collect your stool samples, this should only take you a few minutes each time. Make sure you keep avoiding any foods and/or medications that your doctor instructed until you've collected all your stool samples.

In general, for the gFOBT and the FIT, you will use a dry container from the kit to collect your bowel movement before it goes into the toilet bowl or mixes with urine. Your kit may contain a wooden spatula or brush that you can use to collect a small amount of stool from a couple of different areas of the bowel movement. After you smear the stool on a card, you'll store it in an envelope overnight to dry; alternatively, you may be asked to store it in a container. You can flush the rest of the bowel movement down the toilet.

You will follow this same process for the next two bowel movements you have, as long as they're on different days, unless you're only doing one sample. Once you have collected your samples from three bowel movements, you will mail or bring them to your doctor's office or ship them to a lab to be analyzed for the presence of blood.

The reason you usually collect three samples for this test is that if you have polyps or cancer, the bleeding may not be continuous. Using three samples from three different days gives the test a better chance of catching the bleeding should this be the case.

Follow the instructions that come with your kit, but generally with the flushable reagent pad or tissue, after you have a bowel movement, you place the pad or tissue in the toilet. If there is blood in your stool, the pad will change color. You do this three times on different days and record any changes you see. Note: Results can be affected by toilet bowl cleaners, so it's best to test using one containing clean water.

Post-Test

Make sure that you get your samples sent to the appropriate place as soon as you can to help ensure your results are as accurate as possible. Your kit will explain how to seal and label what you've collected appropriately.

If you're using the flushable reagent pad, send your results to your doctor to keep on file.

After the Test

Once you've collected your last stool sample or performed your last pad/tissue test, you can resume eating the foods and taking the medications your doctor had you stop, if any.

Interpreting Results

If your stool samples were sent to a lab, you may have to wait for two weeks or longer for your test results, which will be either positive (there's blood in your stool) or negative (there's no blood in your stool).

Assuming it's not a false-positive, a positive gFOBT or reagent pad result means that you have bleeding somewhere in your digestive tract, while a positive FIT result is a little more specific and shows that you have bleeding somewhere in your lower digestive tract.

Though the majority of results are negative, this doesn't always mean you don't have colorectal cancer or polyps. Some polyps and cancers don't bleed, so your test may be "clean" despite the presence of these issues. This emphasizes the importance of doing this test annually or as your doctor recommends.

Remember, too, that a positive test result doesn't necessarily mean you have cancer; something else could be going on in your digestive system like hemorrhoids, benign polyps, an ulcer, colitis, or diverticulosis, or you may have swallowed blood that's due to bleeding in your nose or gums.

If you used a flushable reagent pad or tissue, your kit will explain what color change to watch for and how to record the results. Know, however, that results can be affected by blood that's in your urine or from your menstrual period. If your results seem unclear, talk to your doctor about doing a gFOBT or a FIT instead.

If you have any questions about your test results, be sure to talk to your doctor.

Follow-Up

If you had an FOBT to screen for colorectal cancer and your test results are negative, your doctor will probably recommend that you have the test again in a year as long as you are at average risk. Your doctor may also order a colonoscopy as another method of screening for colorectal cancer to ensure that your results aren't, in fact, false-negative.

For people with an average risk for colorectal cancer, a colonoscopy is needed at least every 10 years. But if you have a higher risk of colorectal cancer due to factors such as a family history of colorectal cancer, being obese, using alcohol excessively, or smoking, your doctor will likely want to use screening tools like the FOBT and colonoscopy more often.

If your stool samples test positive for blood, your doctor will order a colonoscopy to investigate your digestive tract further and find the cause and location of the bleeding. This will likely be scheduled as soon as possible. These tests can diagnose many of the conditions that could be the cause of your bleeding, including ulcers, hemorrhoids, colitis, diverticulosis, polyps, and cancer.

If you're diagnosed with ulcers, hemorrhoids, colitis, or diverticulosis, your doctor will talk to you about your treatment options. If polyps and/or cancer are found during your sigmoidoscopy or colonoscopy, your doctor will likely take a tissue sample to check it for cancer or possibly even remove any polyps that are found at the same time as your procedure.

A Word From Verywell

The FOBT is an important screening tool in the fight against colorectal cancer. FOBTs have repeatedly been shown to reduce the number of deaths from colorectal cancer by 18% to 33%.

If you're 45 years old and your risk for colorectal cancer is average, doing an FOBT every year, or as your doctor instructs, can help detect this common cancer early and improve your overall outlook should you develop it. This test is fairly low-cost, easy to perform, doesn't take much time, and it could end up saving your life.

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