To screen for bleeding from the gut/intestine, which may be an indicator of bowel cancer
Faecal Occult Blood Test and Faecal Immunochemical Test
The test is used to screen for bowel cancer especially as part of the National Bowel Cancer Screening Programme. Outside of the Screening Programme the test is not widely available but it may sometimes be used when a patient has a family history of cancer of the intestine and some other organs, or to help diagnose long-term unexplained anaemia if imaging tests, such as colonoscopy, are unavailable. Patients with symptoms that are suspicious of gut cancer will usually be referred for imaging tests without having the FOB test first.
One or more samples of faeces (stool)
For the guiac-based FOB test (gFOBT), you may be instructed to avoid certain medications and follow certain dietary restrictions for several days before collecting the stool samples. For the immunochemical FOB test (iFOBT, FIT), there are no dietary or drug restrictions.
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How is it used?
The main use for the FOB test is as a screen for early bowel cancer. Blood in the stool may be the only symptom of early cancer. If the cancer is detected before it metastasises (spreads to other areas), there is a greater chance it will be cured. A positive FOB test does not necessarily 'prove' that you have cancer. There are other possible causes of bleeding and other follow-up procedures will need to be done to find the source of the bleeding.
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When is it requested?
Faecal occult blood tests may be done as part of a routine examination, possibly annually after age 50, or as directed by your doctor. Most people who have them done do not have any symptoms. They are used mainly as a screening tool for early detection of bowel cancer. The FOB test is used in the National Bowel Cancer Screening Programme. This programme offers screening every two years to all men and women aged 60 to 74 in England, Wales and Northern Ireland (50 to 74 in Scotland). People within the age range are automatically sent an invitation and test kit.
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What does the test result mean?
The FOB test is normally negative. A positive result indicates either blood in the stool or an interfering substance. More than one sample is done to: 1) detect bleeding that happens now and again; and 2) help rule out false positives.
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Is there anything else I should know?
False positives for the FOB test fall into three classes:
- substances or conditions that cause bleeding, such as bleeding gums following a dental procedure or drugs that may cause gastrointestinal bleeding, such as anticoagulants, aspirin, steroids, and large doses of iron preparations;
- other sources of haemoglobin, such as eating red meat within three days before testing; and
- other substances that will react with the FOB test, such as eating fish, turnips, horseradish, or drugs such as colchicines and oxidising drugs (for example, iodine and boric acid). Your doctor may ask you to stay off your medications and follow dietary restrictions before collecting the FOB test samples.
False negatives may be caused by large doses of vitamin C and by not collecting multiple samples (because many conditions only produce blood now and again).
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What kind of procedures might follow a positive FOB test?
Before or after you have an FOB test, you may have a rectal examination. A positive FOB test may be followed by imaging tests to determine the source of the blood:
- Sigmoidoscopy: an examination of the rectum and lower colon (part of the intestines) with a lighted instrument to look for abnormalities, such as polyps
- Colonoscopy: a more thorough examination of the rectum and entire colon
- A double contrast barium enema: a series of X-rays of the colon and rectum
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Should I buy and use self testing FOB kits which are available in some chemists?
The devices available in chemists detect blood in faeces using several different methods. Their sensitivity to detecting blood can vary and without the skills of either an experienced laboratory or doctor the results of these tests can be misleading. Consult your GP if you are concerned and think you need a test performed.