Faecal Occult Blood Test and Faecal Immunochemical Test

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.