Faecal Occult Blood (FOB) test

Faecal occult blood (FOB) test works by detecting tiny amounts of blood which cannot normally be seen in bowel motions.

FOB tests use an analytical system which is based either on a guaiac peroxidase reaction or on an immunological detection system.

  • The guaiac tests detect the presence of blood using the pseudo-peroxidase activity of the haem in haemoglobin. Haem, whether free or bound to a protein such as globin or myoglobin, releases oxygen from peroxide facilitating the oxygenation of a chromogen to give a coloured product. Dietary restrictions in the form of avoiding red meat, any blood-containing food, cantaloupe, uncooked broccoli, turnip, radish, or horseradish  is needed for 3 days prior to the test.
  • Immunochemical tests are more specific since they use antibodies raised against the globin moiety of human haemoglobin and therefore only detect intact human globin or its very early degradation products. No dietary restriction is required for these tests.  Immunochemical tests are also more clinically sensitive than guaiac tests

NHS bowel cancer screening program advises the participant to smear stool on to FOB kit spots from two separate parts of specimen on three separate days. The test is considered positive if 5-6 spots are positive.


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