Fecal Occult Blood Test
The Test Sample
What is being tested?
How is the sample collected for testing?
With gFOBT, your doctor or laboratory will give you one or more test cards. You collect a separate stool sample from three different bowel movements, usually on consecutive days. Each stool sample should be collected into a clean container and should not be contaminated with urine or water. The card is labeled with your name and the date; then, with an applicator stick, you apply a thin smear of stool onto a designated area on the card and allow it to dry. Once it is dry, it is stable. Usually, you will collect all of the consecutive samples, either on a single three-sample card or three separate cards, then return the card(s) to your doctor/laboratory, usually by mailing them.
With the OTC flushable reagent pad/tissue method, you use test pads/tissues that are placed in the toilet after a bowel movement. The pads contain a chemical that produces a color change when blood is present. You watch for the characteristic color change. You may be asked to report the color change on a report form and mail the form to your doctor. This test, like the gFOBT, is usually done on three consecutive days.
With the immunochemical tests, the collection method may vary based on the specific manufacturer. A common approach is to use a special long-handled brush or other device to collect a sample from the surface of the stool. You then use the brush or device to transfer the sample to a special collection card. After the card is allowed to dry, you return the card to your doctor or laboratory. Typically, two or three such samples are collected on different days and sent in one mailing.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
The gFOBT tests and the OTC flushable tests are able to detect any blood that enters the digestive tract. Therefore, any steps you take to avoid introducing blood into your digestive tract will increase the quality of the test sample. Blood that arises from bleeding gums (following dental procedures or gum disease) may be detected by these tests. Bleeding in the stomach that may be triggered by use of drugs such as aspirin will be detected by these tests. Therefore, you are generally advised to avoid taking aspirin or having any dental procedures up to three days before beginning to collect stool samples.
The guaiac-based gFOBT and the OTC flushable pad/tissue rely on a chemical reaction to produce the color change that gives a positive test. Foods such as red meat, broccoli, turnips, and horseradish, and drugs such as colchicine and oxidizing drugs (like iodine and boric acid) may also trigger the same chemical reaction and make the test appear positive even in the absence of blood (a false positive result). Vitamin C, on the other hand, interferes with the chemical reaction and prevents the color formation that should occur when blood is present (a false negative result).
Either your doctor will provide a list of foods and drugs to avoid or a list will be included in the test kit with the instructions for performing the tests. Be sure to check with your doctor before stopping any drugs to be certain that it is safe to do so.
