There are two major types of Faecal Occult Blood Tests (FOBTs), chemical tests (gFOBTs)which detect faecal haem which perioxidises the reagent guaiac, and immunological tests (FITs)which detect faecal globin. As blood passes through the gastrointestinal tract the globin is digested so FITs are negative during upper gastrointestinal bleeding, but gFOBTs may remain positive. Chemical tests are 2-3 times less sensitive than immunological tests for detecting blood in the stool, and also less specific as other peroxidases in the diet may cause false positive gFOBTs. Chemical tests also require sampling from at least 3 consecutive stools to improve their sensitivity whereas FITs are sensitive with just 2 stool samples. For these reasons the National Bowel Screening Programme (NBCSP) uses the immunological test as the FOBT of choice. Guaiac based tests are still used in the Australian Rotary Health programme and in Bowelscan testing.
If a patient has a positive FIT as a participant in the NBCSP, the recommended test is a colonoscopy. An upper endoscopy is not necessary as this test does not detect bleeding from the UGIT.