Overview of Fecal Occult Blood Test (FOBT) | Oncology

Fecal occult blood test (FOBT)

What is a Fecal occult blood test?

The fecal occult blood test (FOBT) is a laboratory test used to check stool samples for occult blood.

Occult blood in the stool can indicate colon cancer or polyps in the colon or rectum, although not all cancers or polyps bleed.

Occult blood is usually transmitted in such small amounts that it can only be detected through the chemicals used in a fecal occult blood test.

If blood is detected by a fecal occult blood test, additional tests may be needed to determine the source of the bleeding. The fecal occult blood test can only detect the presence or absence of blood; you cannot determine what is causing the bleeding.

A stool occult blood test is not recommended if you have symptoms of colon cancer. If you notice blood in your stool or on the toilet, or if you experience abdominal pain or a change in your bowel habits, make an appointment with your doctor.

Types of fecal occult blood tests

Currently, there are 2 types of FOBT:

Guaiac-based FOBT: This test is if by your hospital’s office or a laboratory and is done at home. During the test, he places a stool sample on a test card covered with a plant substance called guaiac. The card changes color if there is blood in the stool. Then you return the card to your doctor’s office or laboratory for interpretation.

Some guaiac-based FOBTs use disposable pads instead of a card. They are available without a prescription at many pharmacies. The results are available to the user immediately.

Immunochemical FOBT: This test uses a particular protein called an antibody. This specific protein attaches itself to hemoglobin, the part of red blood cells that carries oxygen.

The immunochemical test has some profits over the guaiac test. But both tests are used and can provide information about blood in the stool.


The fecal occult blood test is an option for the detection of colon cancer. It may be a choice if you have a regular risk of colon cancer and don’t have any symptoms. The fecal occult blood test is usually repeated annually.

How must I prepare for the fecal occult blood test?

Stool occult blood test results are greatly affected by how you prepare for the test, so it is important to follow the instructions carefully.

Because sure foods can alter the test results, a special diet is often recommended for 48 to 72 hours before the test. The next foods should be avoided during that time:

  • No raw fruits
  • No raw vegetables
  • No red meat; you can eat chicken and pork
  • Less than 250 mg per day of vitamin C-fortified foods or beverages in the 72 hours before the test

How is a fecal blood test done?

For fecal occult blood tests, several (usually three) stool samples are collected for analysis. The reason for testing multiple samples is that bleeding from cancers and polyps is often intermittent and only one of the samples can show blood.

There are two types of fecal occult blood tests, 1) chemical and 2) immune.

Chemical tests: For chemical tests, a solution that contains the chemical guaiac and an oxidizing chemical is used. If there is blood in the stool sample, mixing the solution with blood causes the guaiac to turn visibly blue. The blue color is caused by the interaction (promoted by the oxidizing agent) of the heme portion of the hemoglobin molecule, the oxygen-carrying molecule in red blood cells, and guaiac.

Immunological tests: For immunological tests, a stool sample is mixed with a solution that contains an antibody against globin, the protein that is part of the hemoglobin molecule. The antibody combines with a small amount of gold. When the antibody/gold multifaceted binds to globin in the stool, the antibody/gold/globin complex is put out of solution as a visible line on the test strip.

How do I do a fecal occult blood test?

The fecal occult blood test needs the collection of 3 small stool samples. Usually, the samples are a bit of stool collected at the end of an applicator. Stool samples should be taken one day apart because colon cancers can bleed from time to time, rather than constantly.

You can buy fecal occult blood test kits at the pharmacy to test at home, or your doctor can give you the home test during one of your appointments. These tests provide exact instructions.

Stool samples are collected in a clean container and evaluated for color changes on a test card, or by sending the samples, in a special container and envelope, directly to the doctor’s office for analysis. Your doctor can examine the samples under a microscope or with chemical tests.

What happens during a fecal occult blood test?

A fecal occult blood test is a non-invasive test that you can perform at home whenever you want. Your healthcare provider will give you a kit that includes instructions on how to perform the test. There are two main types of fecal occult blood tests: the guaiac smear technique (gFOBT) and the immunochemical method (iFOBT or FIT). Below are typical instructions for each test. Your orders may vary slightly contingent on the manufacturer of the test kit.

For a guaiac slight test (gFOBT), you will most likely need:

  • Collect samples from three separate bowel movements
  • For each sample, collect the stool and store it in a clean container. Make sure the sample does not mix with urine or toilet water.
  • Use the applicator from your test kit to spread some of the stool onto the test card or slide, also included in your kit.
  • Label and seal all your samples as directed
  • Mail the samples to your healthcare provider or lab

For a fecal immunochemical test (FIT), you will furthermost likely need to:

  • Collect samples from two or three stools
  • Collect the sample from the toilet with the unusual brush or other devices that were involved in your kit
  • For each sample, use the brush or device to collect the sample from the surface of the stool
  • Brush the sample onto a test card
  • Tag and seal all your samples as directed
  • Mail the samples to your healthcare provider or lab

Are there any risks to the test?

The fecal occult blood test is harmless and painless.

What should I expect after the procedure?

You can resume your normal activities immediately after FOBT. After learning the results, talk with your healthcare team about the next steps.

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