Also known as: Faecal occult blood test (FOBT)
Name: Faecal occult blood test
Related tests:
Why is the test done?
To detect bleeding in the gastrointestinal tract, which could be a sign of colon cancer.
When is it done?
As part of routine colon cancer screening; it is recommended every year from the age of 50, or as prescribed by your GP on the basis of your family history.
What samples are required?
One or more stool samples.
What is being tested?
The test looks for hidden blood in the stool. The small amounts of blood normally lost in the gastrointestinal tract do not show up in the test. The test is positive when abnormal amounts of blood are lost in the gastrointestinal tract.
How is the sample collected for testing?
Several methods are available for detecting occult blood: the guaiac test (gFOBT), an over-the-counter device based on a colour reaction, and immunochemical methods (iFOBT or FIT). Each method has its own way of collecting the sample and carrying out the test.
The guaiac test uses a card. Stool is collected on three different occasions in a clean container, avoiding contamination with urine and water. Using a special stick, a thin layer of stool is spread on the marked area of the card and left to dry. Once dry, the sample is stable and must be sent to the laboratory.
With the over-the-counter device based on a colour reaction, a test strip is applied to the toilet paper after a bowel movement. The strip contains a chemical reagent that changes colour in the presence of blood. The colour change should be reported to the doctor.
As with the guaiac test, this test is usually repeated on three consecutive days.
Immunochemical methods use different collection methods depending on the manufacturer. Usually a special brush or other device is used to take a sample from the surface of the stool and transfer it to a card. The card is left to dry and examined in the laboratory. Two or three samples are usually collected on different days.
How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?
How is it used?
The faecal occult blood test is mainly used as an early screening test for colon cancer. Blood in the stool may be the only symptom of early-stage cancer. The chances of the cancer being curable increase if it is diagnosed early. The faecal occult blood test does not diagnose cancer; other procedures are needed to find the source of the bleeding, because blood may point to other gastrointestinal problems.
The faecal occult blood test can also be used to look for the cause of anaemia, such as a bleeding ulcer. It may be requested when there are symptoms and signs of anaemia, such as tiredness, low haemoglobin and haematocrit and/or abnormally dark stools.
When is it requested?
The faecal occult blood test is often one of the routine tests. It is mainly requested as a screening test for the early diagnosis of colon cancer. It is recommended every year after the age of 50, or as prescribed by your GP on the basis of your family history. Most people who have this test have no symptoms.
It may also be requested to look for the cause of anaemia that could be due to gastrointestinal bleeding.
What does the test result mean?
The faecal occult blood test is normally negative. A positive result indicates abnormal bleeding from the gastrointestinal tract. This blood loss could be due to ulcers, diverticulosis, bleeding polyps, inflammatory bowel disease, haemorrhoids, swallowing blood from bleeding gums or after a nosebleed, or benign or malignant tumours.
What else should I know?
The occult blood test can sometimes give false-positive results. Depending on the method used, you should avoid:
1) substances or situations that cause bleeding, such as dental procedures, or medicines that can cause gastrointestinal bleeding, such as anticoagulants, aspirin, steroids and large doses of iron preparations.
2) other sources of haemoglobin, such as eating red meat in the three days before the test.
3) other substances such as fish, turnips and horseradish, or medicines such as colchicine, or oxidising substances (e.g. iodine and boric acid). Your GP may ask you to stop current treatments and follow certain dietary restrictions before collecting the samples for the occult blood test.
Bleeding from polyps and tumours is intermittent, so blood is not evenly distributed across all samples. Collecting three samples on three different days increases the chances of detecting intermittent bleeding.
1. Is the over-the-counter home test really any good?
2. What further tests should be done if the result is positive?
3. Can other tests for colorectal cancer be done on the stool sample?
1. Is the over-the-counter home test really any good?
The over-the-counter test is fairly sensitive, but guaiac tests and immunochemical methods are preferred. Reading the colour change is subjective, whereas a laboratory test is more reliable. Compared with the patient, laboratory staff have more experience in interpreting colour changes, which are sometimes subtle, and are more familiar with the causes of false-negative or false-positive results.
2. What further tests should be done if the result is positive?
A faecal occult blood test should be preceded or followed by a rectal examination. If the result is positive, the following should be carried out:
sigmoidoscopy: an endoscopy of the rectum and the last part of the colon to detect abnormalities such as polyps
colonoscopy: a more thorough endoscopy of the rectum and the whole colon; and
double-contrast barium enema: a series of X-rays of the colon and rectum
3. Can other tests for colorectal cancer be done on the stool sample?
Yes. Stool DNA testing is available. It can detect certain DNA changes in cancer cells shed in the stool. The test has acceptable sensitivity, although it cannot identify all types of colorectal cancer. Recent guidelines (March 2008), issued jointly by several scientific societies, added the stool DNA test to the list of tests recommended for colorectal cancer screening.