Fecal Occult Blood Test

Medically Reviewed By William C. Lloyd III, MD, FACS
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What is a fecal occult blood test?

Despite its weird name, fecal occult blood testing (FOB testing or FOBT) is an important clinical test in gastroenterology. The word "occult" comes from the Latin for "hidden." The occult blood test simply looks for traces of blood and metabolized blood substance in the stool that can’t be seen with the naked eye. The fecal occult blood test—also called a guaiac or Hemoccult® test—is used as a diagnostic tool for detecting abnormal bleeding inside the digestive tract. If the FOBT finds occult blood in your stool, then your doctor will follow up with additional testing to determine where the blood came from and how the bleeding should be treated.

Why is a fecal occult blood test performed?

Your doctor may recommend a fecal occult test if he or she suspects you might be experiencing bleeding within your gastrointestinal tract. That bleeding could be due to hemorrhoids, diverticulosis, intestinal polyps, or colorectal cancer—among other causes. Your doctor may ask you to have a fecal blood test if:

  • You cannot have a colonoscopy to screen for colon cancer or rectal cancer, or you decline a colonoscopy
  • Your stool looks abnormal, especially if it is black or tarry
  • You exhibit symptoms of a gastrointestinal disorder like diverticulosis or an ulcer that can cause bleeding in the digestive tract
  • You have a family history of colorectal cancer and require frequent screening
  • You have an unexplained case of anemia

Current colon cancer screening guidelines recommend everyone older than 50 have a colonoscopy, which looks for both cancerous tumors and noncancerous polyps. People of average risk should have a colonoscopy every 10 years in the absence of cancer. People with such risk factors as family history of colon cancer, obesity or being overweight, and smoking or misusing alcohol may require more frequent screening. In those cases, some doctors will recommend routine fecal occult blood tests to screen for blood in the stool instead of frequent colonoscopies.

How is fecal occult blood testing performed?

Fecal occult blood testing is not invasive and it doesn’t hurt. Depending on the type of test kit you have, you will need to:

  • Collect stool from three separate bowel movements.
  • Flush the toilet multiple times before beginning, as toilet bowl cleaner residue can affect the test results. If a collection drape is not included in the kit, loosely cover the toilet bowl with plastic wrap so the stool sample never touches the water.
  • Use the provided stick, paddle or brush to obtain a sample of your stool as directed.
  • Place the sample in the provided sterile container, or smear the sample onto the provided test card. You should have three containers or cards for three separate samples.
  • Label all samples clearly as instructed.
  • Mail or deliver the samples to the lab or to your provider’s office.

What are the risks and potential complications of fecal occult blood testing?

You may find the idea of collecting a stool sample unpleasant, but you will not experience any physical risks or complications from the procedure. To make the process easier, wear vinyl gloves and/or a face mask to avoid coming in contact with the stool and to avoid any odors.

How do I prepare for fecal occult blood testing?

Ask your doctor for specific instructions regarding how to prepare for the FOB test. Some providers recommend you make certain dietary changes or follow other guidelines. In general, to avoid triggering a false-positive test:

  • Avoid taking a vitamin C supplement for 72 hours prior to collecting a stool sample.
  • Avoid eating red meat for 72 hours prior to sample collection.
  • Avoid eating melons, turnips, radishes and horseradish for three days prior to testing, as compounds in these fruits and vegetables can affect the test results.
  • Do not collect a stool sample if you have actively bleeding hemorrhoids or if you are on your menstrual period, as either of these situations can introduce blood to the sample and cause a false-positive result.
  • Stop taking nonsteroidal anti-inflammatory medications like aspirin at least one week before you begin collecting the stool samples. If you take prescription anticoagulants (‘blood thinners’), continue to take them as normal unless advised otherwise by your doctor.

Questions to ask your doctor

Because the precise test procedure varies by manufacturer, be sure to ask your doctor about how to collect the stool sample. Obtaining an uncontaminated sample is crucial for getting accurate test results. You may want to ask your doctor:

  • Why are you recommending this test to me?
  • How will the results help you arrive at a diagnosis? What does blood in the stool mean?
  • Do I need to make sure the stool sample does not touch the toilet bowl water?
  • Do I need to stop taking any medications or supplements prior to collecting the sample?

What can I expect after a fecal occult blood test?

This simple, noninvasive test has no known side effects or complications, and you will not experience any down time after collecting your stool samples. After you have obtained the third sample, you can go back to eating and drinking normally, as well as taking vitamin or mineral supplements and over-the-counter pain relievers.

When should I call my doctor?

Your doctor likely will mail you a letter with your test results. If you have a positive fecal occult blood test, you will need to have further testing to determine what is causing your digestive tract bleeding. Your provider will let you know if you need to make a follow-up appointment.

If your test is negative, you can enjoy the peace-of-mind that comes from knowing you do not have any active bleeding inside your digestive tract. A positive test result might help your doctor identify a condition like colorectal cancer or diverticulosis early, leading to faster and possibly more effective treatment.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Aug 27
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
  1. Fecal Occult Blood Test. Mayo Clinic. https://www.mayoclinic.org/tests-procedures/fecal-occult-blood-test/about/pac-20394112
  2. Fecal Occult Blood Test. MedlinePlus, U.S. National Library of Medicine. https://medlineplus.gov/labtests/fecaloccultbloodtestfobt.html
  3. Tests to Detect Colorectal Cancer and Polyps. National Cancer Institute. https://www.cancer.gov/types/colorectal/screening-fact-sheet
  4. Fecal Occult Blood Test. Harvard Health Publishing. https://www.health.harvard.edu/bladder-and-bowel/fecal-occult-blood-test-