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Guaiac and immunochemical FOBT may identify more accurately individuals with colorectal cancer

Use of a simple test, in addition to traditional screening tests, identifies more accurately individuals who might have colorectal cancer.

This test, known as an immunochemical faecal occult blood test ( FOBT ), when used for individuals who have previously tested positive on initial screening with the more traditional guaiac FOBT, "could decrease substantially the number of false positives in a screening programme for colorectal cancer", according to lead author Callum G Fraser, and could reduce the need for colonoscopy in many individuals.

The guaiac-based FOBT, which detects blood ( haemoglobin ) in the faeces, is currently used to screen individuals for colorectal cancer. However, although this test is cheap and relatively easy to do, it is not very accurate and has a low clinical sensitivity and specificity since cancer is not the only cause of a positive result. Nevertheless, individuals who are positive in this test are followed-up by colonoscopy, a more expensive procedure that has some associated risks.

Fraser and colleagues proposed that individuals who are only weakly or moderately positive in the guaiac test could be selected further by use of an immunochemical test, thereby distinguishing individuals who are less likely to have cancer, and thus sparing them the additional colonoscopic examination.

A total of 1486 participants who were guaiac FOBT positive were invited to participate, and around 50% returned samples for testing with the immunochemical test.
Of these individuals, 173 ( 22% ) tested negative in both samples provided ( N/N ), 129 ( 16% ) tested positive in one of two samples ( N/P ), and in 498 ( 62% ) participants both samples were found to be positive ( P/P ).
Only one patient in each of the N/N and N/P groups had colorectal cancer compared with 38 ( 8% ) of the P/P participants. The odds ratio for P/P participants having cancer was 7.57 compared to a reference group of N/N and N/P individuals.

" Although immunochemical FOBT is more expensive than guaiac FOBT, its use in testing those who are weak positive on initial guaiac FOBT would reduce the need for colonoscopy by about 30%. The implications for national screening programmes are important in terms of reducing costs, inconvenience, and associated morbidity, without compromising the effectiveness of screening". Fraser said.

Source: The Lancet, 2006