Bleeding into the gastrointestinal tract may be rapid with the vomiting of blood (haematemesis) or the passage of blood through the rectum (melaena).
When the bleeding is chronic with only small amounts of blood being passed in the faeces, the blood (or its breakdown products) is not recognized in the faeces and is referred to as occult (hidden)
blood.
When is Occult blood requested
Requests for occult blood testing are usually made to investigate the cause of iron deficiency anaemia or to assist in the diagnosis of bleeding lesions of the gastrointestinal tract, e.g. peptic ulcer,
carcinoma, or diverticulosis.
Methods applied in Occult blood Test
Chemically using guaiac based reagents prepared in the laboratory, e.g. aminophenazone test, or ready-made reagent in kit tests, e.g. Hema-Screen.
Immunologically using a haemoglobin specific cassette or strip test such as the Instant-View
Immunological tests are more expensive
Evaluation of Occult blood test
Specificity of occult blood tests:
Chemical tests are not specific for haemoglobin. They are based on the principle that haemoglobin and its derivatives react in a similar way to peroxidase
enzymes, i.e. they catalyze the transfer of an oxygen atom from a peroxide such as hydrogen peroxide to a chromogen
such as guaiacum, 2,6-dichlorophenolindophenol or aminophenazone. Oxidation of the chromogen is shown by the production of a blue, blue-green, or pink colour.
In chemical tests, non-haemoglobin substances with peroxidase activity can therefore cause false positive reactions.
Other substances can interfere with peroxidase activity resulting in false negative results. The specificity of chemical tests can be improved by dietary restriction.
Immunological occult blood tests are specific for the detection of human haemoglobin in faeces. A monoclonal antibody directed against human haemoglobin is used. This
selectively binds to human haemoglobin present in the faeces.
Sensitivity of occult blood tests:
Considerable variation of
sensitivity is shown by both chemical and immunological occult blood tests. Highly sensitive tests can be misleading because they detect trace amounts of blood which can be found in normal faeces. Highly sensitive chemical tests can give false positive reactions when faeces contain dietary substances which have peroxidase-like activity. Tests of low sensitivity can also be misleading because they may fail to detect small
amounts of blood which are pathological.
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