Occult blood faeces

Keywords: FOB, Faecal occult blood, Haemoglobin faeces

Specimen:

Approximately 10 g freshly collected faeces.

Gastrointestinal tract bleeding may be intermittent so specimens should be taken from three successive bowel motions and forwarded to the laboratory in separate containers.

Greater sensitivity is achieved by sampling the stool surface.

Immunochemical methods are preferred.

For the haemperoxidase assay, appropriate diet prior to collection is required (see below).

Method:
  1. Pseudoperoxidase (haemperoxidase). A special diet is required for at least 48h, with exclusion of red meat and certain raw fruits and vegetables, especially melon, carrots and radish.

    Iron supplements and excessive amounts of vitamin C should also be avoided for 48 h prior to collection.

  2. Immunochemical. No special diet is required.

For both methods, the patient should avoid excessive alcohol or drugs which may cause gastro-intestinal bleeding (eg, aspirin, NSAID, corticosteroids).

Reference Interval:

Not detected.

Application:

Screening test for colorectal carcinoma. These tests are not appropriate for the detection of upper GIT bleeding.

Interpretation:

Immunochemical methods are preferred, as no special diet is required and they are more specific for lower GIT bleeding.

With haemperoxidase methods false positive results can occur if the instructions as to diet and drug exclusion are not followed or if there is another source of GIT bleeding.

False negative results can occur if high doses of vitamin C have been ingested.

Quantitation of faecal blood loss is rarely indicated and is only performed in a few specialised laboratories.

Reference:

Pignone M et al. Ann Intern Med. 2002; 137(2): 132-141.