Question

1. Explain the principle of the Guaiac Test for occult blood and the reasons that guaiac is the reagent of choice.

2.    Write the instructions that must be given to patients as regards the collection of stool specimen for occult blood test, including the dietary restrictions.

3. What is the most sensitive and specific method for the detection of occult blood?

Answer 1

1.PRINCIPLE OF GUAIAC TEST

• This test is based on oxidation of phenolic compounds present in guaiac to quinones result in the production of blue colour.
• When fecal specimen containing occult blood is applied to test paper,contact is made between haemoglobin and guaiac.
• A pseudoperoxidase reaction will occur upon addition of developer solution with a blue chromagen formed propotional to colour of haemoglobin.

Guaiac reagent is a natural resin extracted from wood of the Guaiacum officinale plant.Colour change in this test occurs as guaiaconic acid is oxidised by hydrogen peroxide to blue quinone in presence of haem derived from blood.

2. Avoid taking the following 3 days before the test:

• Red meat like beef or lamb.
• Raw fruits and vegetables.
• Vit c and their supplements.
• Antacids and NSAIDS.
• Iron supplements.

Blood should not be collected during -

• Menstruation.
• Bleeding piles.
• Blood in urine.

3. Immunochemical fecal occult blood test is most sensitive and specific method for occult blood test. No dietary restrictions are needed and can be done on random sample.