Fecal Occult Blood Test (FOBT): About This Test

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What is it?

A fecal occult blood test checks for hidden (occult) blood in the stool. You place a small sample of stool on a chemically treated card. Then a special chemical solution is put on top of the sample. If the card turns blue, there is blood in the stool sample.

Why is this test done?

A fecal occult blood test is done to check for colorectal cancer and other types of gastrointestinal problems. These include hemorrhoids, anal fissures, and colon polyps, which can cause blood in the stools.

How can you prepare for the test?

Before you do a fecal occult blood test, avoid the following for 2 to 3 days before the test:

  • Eating turnips, beets, radishes, horseradish, artichokes, mushrooms, broccoli, bean sprouts, cauliflower, apples, oranges, bananas, grapes, and melon. These foods can cause the test to be positive for blood when blood is not in the stool.
  • Eating red meat. This may cause false test results. Small amounts of chicken, turkey, or fish will not affect the test.
  • Taking iron supplements
  • Taking aspirin (or products that contain aspirin) and non-steroidal anti-inflammatory drugs (NSAIDs). Also avoid taking other medicines that irritate the stomach or intestines.
  • Taking vitamin C supplements

Do not do the test during your menstrual period or if you are having bleeding from hemorrhoids. And don't test a stool sample that has been in contact with toilet bowl cleaning products that turn the water blue.

What happens during the test?

You can check for blood in your stool at home. There are different types of home tests you can use. Make sure to follow the instructions that come with any test. For most tests, you will use stool samples from three different bowel movements over three different days.

General instructions

For any home test, follow these guidelines:

  • Check the expiration date on the package. Do not use a test kit after its expiration date. The chemicals in the kit may not work properly after that date.
  • Store the test kit as instructed. Many kits need to be stored in a refrigerator or cool place.
  • Read all of the instructions for your test closely before doing the test. Be aware of any special things you need to do before the test. This may include not eating certain foods or limiting your physical activity.
  • Follow the instructions exactly. Do all the steps in order, without skipping any of them.
  • If a step in the test needs to be timed, use a watch. Don't guess at the timing.
  • If you are colour-blind or have trouble seeing colours, have someone else read the test results for you. Most test results are colour changes on a test strip.
  • Record the results of the test so you can discuss them with your doctor.

Stool guaiac cards

These instructions are for one of the most common tests used to find blood in the stool.

  • Complete the information on the front of each card.
  • During a bowel movement, collect a small amount of stool on one end of an applicator. You might try catching the stool on some plastic wrap draped loosely over the toilet bowl and held in place by the toilet seat. If you use a container to collect the stool, first clean and rinse it well. This will get rid of any substance that may affect the test results.
  • Apply a thin smear of stool inside box A.
  • Reuse the same applicator to get a second sample from a different part of the stool. Apply a thin smear inside box B.
  • Close the cover of the slide.
  • Complete the remaining two cards in the same way for two other bowel movements.
  • You probably will be instructed to return all slides to your health professional either in person or by mail within 4 days after you collect the samples.
  • If your test kit has the developer solution, wait 3 to 5 minutes before you put 1 drop of this solution on the area with the stool. Put 1 drop of the solution on the control areas of the card so that you will know what positive and negative test results should look like. An area to read the results is found on the reverse side of the card. Turn the card over and read the results within 10 seconds.

What else should you know about the test?

A fecal occult blood test may be used to check for colorectal cancer. But it is never used to diagnose this condition. If a fecal occult blood test finds blood in the stool, you may need more tests to find the reason for the bleeding.

Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor or nurse call line if you are having problems. It's also a good idea to keep a list of the medicines you take.

Where can you learn more?

Go to https://www.healthwise.net/patientEd

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Current as of: October 14, 2016