Fecal Occult Blood Test (FOBT): What You Need to Know

fecal occult blood test

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The fecal occult blood test (FOBT) is a lab test. It is used to check the stool for the presence of any hidden blood. Occult blood in the stool is usually an indicator of colon cancer or polyps in the rectum ore colon. Not all cancers or polyps bleed, but occult blood in the stool can be a sign of colon cancer or polyps.

Occult blood comes in small amounts. This means it can be difficult for the doctor to detect. This is why it is necessary to use certain chemicals during the lab test to detect and determine correct results. Additional tests may be necessary if blood is found during the fecal occult blood test. Unfortunately, the fecal occult blood test is only able to detect blood in the stool and is unable to determine the source of the bleeding. This is why it may be necessary to take additional tests to find out why there is blood.

How to Prepare

Preparing for the fecal occult blood test involves avoiding certain foods and medicines. In order to receive accurate test results, it is important not to eat anything other than what your doctor tells you. Certain foods and medications can affect your results by creating a false-positive or false-negative. The doctor may perceive that there is blood when there really is not any or vice versa. It is likely that your doctor will ask you to avoid broccoli, turnips, red meat, horseradish, vitamin C supplements, aspirin, and ibuprofen or other pain relievers.

What to Expect

Your doctor will discuss with you what you should expect from the fecal occult blood test. There are actually different kinds of tests, each containing separate processes and approaches. Guaiac fecal occult blood test (gFOBT) is a process that involves the patient to collect a stool sample from his or her bowel movement. Then, the patient uses an applicator stick and applies a smear of the sample to an indicated area on a test card. The individual collects the stool on consecutive days. When the samples are dry they are sent to the doctor or a lab for analysis.

Another option is a kit that is available at the store without needing a prescription. The kit comes with a pad or tissue, which you place in the toilet bowl after a bowel movement. You will notice if there is blood in your stool because you will notice the color changes in the pad. Report any changes that you see or notice to your doctor.

A third option is called the immunochemical fecal occult blood test (iFOBT or FIT). For this method, the patient usually uses a certain spoon or device to collect a sample of the stool. Then, the patient stores the sample in a container. This container comes with the test kit. The patient gives the container to a doctor or a lab. Unlike iFOBT, gFOBT requires dietary restrictions before the stool sample can be collected.

Risks and Benefits

There are a few risks or limitations associated with the fecal occult blood test. For example, the test is not always accurate. You may receive a false-negative result. It can also show a false-positive if you have bleeding from a different source. Another limitation of the fecal occult blood test is that it only leads to additional testing if you receive a positive result. Further observation and testing is necessary because the FOBT only allows the doctor to detect whether or not there is occult blood in the stool. Lastly, fecal occult blood tests is not able to detect all cancers. However, a colonoscopy is another option for further examination.

Despite having some risks and limitations, fecal occult blood tests do have some benefits. If you are at least 50 years old or are at risk of colon cancer, your doctor can use the FOBT to screen for colon cancer. Another benefit is that it can evaluate possible causes of unexplained anemia. The FOBT can examine and determine if bleeding in the digestive tract is contributing to anemia.


As mentioned previously, the FOBT can lead to a false-negative or false-positive. However, your doctor will review the results and then share them with you. You will receive a negative result if no blood is detected in the stool samples. Even though you may receive a negative result, your doctor may request further additional tests depending on your condition.

On the other hand, a positive result occurs if your doctor finds blood in the stool samples. Your doctor is likely to recommend additional testing to find the source of the bleeding, such as a colonoscopy.