The fecal immunochemical test for hemoglobin is more specific than the stool guiac test for detection of occult blood but is more expensive. It can have false positive and false negative results. It should be used as part of diagnostic strategy that stratifies a patient according to risk of an occult colorectal tumor.
A major advantage of the immunochemical test is that it is more specific for hemoglobin, with fewer false positive from foods with peroxidase activity or ascorbic acid.
Causes of a false negative test (colon adenoma or carcinoma present with negative test result):
(1) nonbleeding adenoma, especially in proximal colon
(2) intermittent, low volume bleeding adenoma or carcinoma
(3) FIT with higher threshold for a positive result
(4) hemoglobin degradation during specimen transport and handling (delayed mailing in hot weather)
Causes of a false positive test (absence of colon adenoma or carcinoma with positive test result):
(1) hemorrhoids
(2) diverticulitis
(3) age > 65 years
(4) smoking
(5) angiodysplasia
The use of anticoagulants (aspirin, clopidogrel, warfarin, etc) may increase false positives if there is increased gastrointestinal bleeding but also can result in more true positives.
A small amount of blood in the diet does not appear to affect FIT since the hemoglobin is digested during bowel transit. However, a large amount of blood together with rapid bowel transit could cause a false positive result in theory.
