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.
The faecal occult blood test (FOBT) or faecal immunological test (FIT) checks for blood in your faeces. Normally, there will not be enough blood lost through the gastrointestinal tract (stomach and intestines) to turn a FOBT positive or for you to notice it by looking. Any significant amount of blood being passed (seen as bloody or dark, tarry faeces) should be investigated.
A positive FOB test will tell your doctor that you have bleeding occurring somewhere in your gastrointestinal tract. This blood loss could be due to ulcers, diverticulitis, bleeding polyps, inflammatory bowel disease, haemorrhoids (piles), from swallowed blood due to bleeding gums or nosebleeds, or it could be due to benign or malignant tumours. Anything that sticks out into the intestine, like a polyp or tumour, and is rubbed against by the faeces as it passes through, has the potential to bleed now and again. Often this small amount of blood is the first, and sometimes the only, symptom of early bowel cancer, making the FOB a valuable screening tool.
How is the sample collected for testing?
First, your doctor may give you some instructions about dietary restrictions that you should follow and drug treatment that you should avoid before collecting the FOB samples. For example, you should not eat red meat for three days before testing. Other substances that could cause a false positive test result include fish, turnips, horseradish, and drugs such as colchicines and oxidizing drugs (for example, iodine and boric acid). Please follow your doctor's instructions carefully.
You collect tiny faeces samples yourself, in the privacy of your own bathroom, using a special kit.
The main method for faecal occult blood testing is the guaiac smear method (gFOBT), explained below.
Your doctor or laboratory will typically give you three 'cards' (or one card with three flaps) so that you can collect samples from three different stools, usually on different days. Label the card/flap with your full name and the date. It is important that the stool sample is not contaminated with urine or water. Suggestions to catch your sample include any clean disposable container or your hand in a small plastic bag. Use the applicator stick provided to apply a thin smear of stool onto each filter paper square/window contained on the card, and allow it to dry. Usually you will collect all three samples, then return all of them to your doctor/laboratory at the same time, sometimes by posting them.
An alternative method for faecal occult blood testing is the faecal immunochemical test. The collection method varies between manufacturers. Some use a card similar to that described above. Others use a plastic wand to transfer a small amount of faeces to a tube of liquid. Read all the instructions very carefully before starting to collect your sample.
Is any test preparation needed to ensure the quality of the sample?
For guaiac-based FOBT (gFOBT) special dental, dietary and drug restrictions may be recommended. These tests detect any blood that enters the digestive tract. Therefore, steps that are taken to avoid introducing blood into the 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. You should avoid having any dental procedures up to three days before beginning to collect stool samples.
- Avoid collecting samples during a nenstrual period.
- Bleeding in the stomach that may be triggered by use of non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, naproxen, and ibuprofen. Avoid taking these drugs for seven days prior to testing, if clinically possible.
- The guaiac-based gFOBT relies on a chemical reaction to produce the colour change that gives a positive test. Foods such as red meat, broccoli, turnips, cauliflower, apples, oranges, mushrooms, 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 human blood (a false positive result). You may be instructed to avoid these foods and drugs for three days prior to and during the testing period.
- Vitamin C, on the other hand, interferes with the chemical reaction and prevents the colour formation that should occur when blood is present (a false negative result). Vitamin C supplements and fruit juices that contain vitamin C should be avoided for three days prior to and during testing.
Check with the doctor before stopping any drugs to be certain that it is safe to do so.
For immunochemical methods (iFOBT, FIT), there are no dietary, drug, or dental procedure restrictions. However, avoid collecting samples during a menstrual period. The test uses antibodies to detect only human blood from the lower digestive tract (colon).
For all tests, follow the instructions that are provided by the doctor or included in the test kit.
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.
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.
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.
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).
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
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.