FAECAL OCCULT BLOOD TEST
Faecal occult blood test is a test used to detect haemoglobin in faeces or stool which is not detectable macroscopically. Occult means unseen or invisible so we are looking for possible blood in stool that we cannot see. A positive occult blood test usually indicates massive loss of blood in the gastrointestinal tract and this is usually seen in;
- Upper gastrointestinal bleeding
- Colorectal cancer
- Anaemia of unknown cause
- Fever of unknown origin
METHODS
There are two main methods used in the Faecal Occult Blood Tests
CHEMICAL METHOD
Principle
The test is based on the principle that haemoglobin is going to act as a pseudoperoxidase if present in the stool and break down the hydrogen peroxide into water (H2O2) and oxygen radical (O2-). The oxygen radical (O2-) reacts with the guaiac (chromogen) to give a blue colouration which confirms a positive test.
Procedure
A small amount of stool is picked from different portions of the stool and smeared evenly on the pad on both the positive and negative control portions. The pad is impregnated Guaiac which is going to act as a chromogen. A developer solution is then added to the pad. The developer contains hydrogen peroxide (H2O2) which acts as a substrate for the enzyme.
PATIENT PREPARATION PRIOR TO TEST
Patients should collect stool samples under the following conditions
During or until three days after menstrual period
While patient has bleeding hemorrhoids(piles)
When patient has conditions that cause bleeding (e.g. bleeding gums or passing bloody urine
Dietary restrictions
Patients should avoid eating or taking any of the following foods at least three days prior to the test.
Red meat, poultry products, fish or any food containing food because of the myoglobin may cause false positive results.
Raw fruits e.g. apples, banana, oranges etc.
Vegetables e.g. beets, broccoli, carrots, mushrooms, cucumbers, etc. because they also contain pseudoperoxidase and may cause false positive results.
Drug restrictions
The following drugs should be avoided at least three days prior to the test
Avoid or withdraw alcohol, aspirin, and other gastric irritants including NSAIDS because they may cause intestinal bleeding and cause false positive result.
Other restricted medicines include colchicines, reserpine, barium sulphate, cough medicines, corticosteroids, anticoagulants etc.
Vitamins, including iron supplements (blood tonics) and vitamin C or vitamin C enriched foods because Vitamin C has antioxidant properties and mops up the oxygen free radicals. As a result, there will be oxygen radical to react with the chromogen to produce a colour change producing false negative results.
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