Microbiology Laboratory

OCCULT BLOOD TEST (OBT)

Written by MicroDok

AIM: To detect the presence of occult (hidden) blood in stool specimens as an aid in the diagnosis of infections that cause bleeding lesions of gastrointestinal tract (GIT), e.g. peptic ulcer, carcinoma, or diverticulosis.

MATERIAL/APPARATUS: Stool specimen, HEMA screen test paper, HEMA screen developing solution, timer. NOTE: Both HEMA screen test paper and HEMA screen developing solution are components of HEMA SCREEN test kit used for occult blood test. Other methods of detecting occult blood in stool specimen include: aminophenazone test, and the immunological test using ready – made reagents in kit tests.

PRINCIPLE: The principle behind the use of HEMA SCREEN test kit in the detection of occult blood in stool specimen is thus: the HEMA SCREEN test paper is made from a plant (tree) called Guaicidium offinacle, and this paper contains a component called guaiacum or guaiac – a chromogen. Haemoglobin (a component of blood) reacts in a similar way to peroxidase enzymes, i.e. they catalyze the transfer of an oxygen atom from a peroxidase such as hydrogen peroxide (H2O2) to a chromogen such as guaiacum, 2, 6 – dichlorophenolindophenol or aminophenazone. Oxidation of the chromogen is shown by the production of a blue – green, or pink colour.

METHOD/PROCEDURE:

  1. Collect a portion of the stool specimen using the applicator stick. Make sure that you collect from the middle of the specimen and not just from the surface only.
  2. Open the HEMA SCREEN test paper and spread the stool specimen collected on the test area provided. Ensure that only a thin smear is made on the test area.
  3. Add 2 drops of the HEMA SCREEN peroxidase developing solution onto the smeared stool specimen on the test area.
  4. Cover the test paper and press with your hand, and open the reverse side of the test paper after 30 seconds to read the result

REPORTING OF THE RESULT:

Presence of a blue – green colouration is a positive result, and it shows that the stool specimen contains occult blood. Absence of a blue – green colouration is a negative result, and it shows that the stool specimen does not contain occult blood.

NOTE: Occult blood test (OBT) is very important because it detects occult/hidden blood which occurs in the GIT when bleeding is chronic (slow), and blood is passed into the feaces in small amounts that cannot be detected. This blood is not recognized when looking at the feacal sample physically, and is thus called occult (hidden) blood. That is why OBT is needed to detect such blood and its breakdown products that pass into the feaces undetected.

It is also very important to tell the patient to deviate from eating vegetables and meat for at least two days prior to collecting the stool specimen. This is because the duo (meat and vegetables) contain peroxidase and thus will react with the test paper to give a false positive result.

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MicroDok

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