Faecal Occult Blood (FOB) refers to blood in the stool (faeces) that is not visible. A faecal occult blood test (FOBT) checks for this blood by smearing faeces on a test strip.
The old guaiac tests such as Hemoccult® test for the heme component in haemoglobin as it has a peroxidase-like effect, rapidly breaking down hydrogen peroxide. Guaiac tests need certain foods to be eliminated from the diet to avoid false positive results, including meat and vitamin c, and generally lack sufficient sensitivity and specificity to make them a accurate test. An improvement on the old guaiac test is the more sensitive Hemoccult SENSA®.
Newer improved tests
Newer improved tests such as the immunochemical Faecal Occult Blood Test (iFOBT) used by National Bowel Cancer Screening (Figure 1) are highly sensitive, and superior to the non-immunichemical Guaiac tests. These tests use specific antibodies to detect globin and have an accuracy of greater than 80% (sensitivity and specificity). In 2015 the iFOBT test will be mailed out to all Australians every 2 years turning 50, 55, 60, 65, and 70. By 2010 those turning 74 will be included.
WHY DO iFOBT?
The iFOBT will detect small amounts of blood within faeces not visible with the naked eye. A positive FOBT implies bleeding from the gastrointestinal tract, anywhere from the mouth to the rectum, with the colon the commonest site. It is used as a screening test in a low risk healthy population.
An estimated 1-5% of tested populations have a positive iFOBT. Those with a positive iFOBT should be referred to a gastroenterologist or colorectal surgeon for a colonoscopy to exclude polyps or cancer of the colon.
If a person has a persistent altered bowel habit (greater than 2 weeks), rectal bleeding, or a strong family history of colon cancer, or previous polyps then they are not low risk, and should consult a gastroenterologist or colorectal surgeon to organise a colonoscopy. The iFOBT test is only a screening test for low risk patients without symptoms.
At what age should FOBT be performed and how often?
The National Bowel Cancer Screening Australia and Bowel Cancer Australia guidelines suggest routine iFOBT screening every 1-2 years from the age of 50 or 10 years earlier than the youngest family member previously diagnosed with colorectal cancer.