Guaiac-based fecal occult blood tests (G-FOBT) significantly underestimate the prevalence of advanced adenomas and colorectal cancer in screening populations, compared to immunochemical fecal occult blood tests (I-FOBT), according to a report in the July issue of Gastroenterology.

Dr. Leo G. van Rossum from Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands told Reuters Health, "I-FOBT performs better in colorectal cancer screening than the G-FOBT even without considering the obvious benefit of being able to change the cut-off value of the I-FOBT. Therefore, countries that start screening for colorectal cancer should implement the I-FOBT as the primary screening test."

Dr. van Rossum and colleagues sent either the Hemoccult II G-FOBT (Beckman Coulter) or the OC-sensor I-FOBT (Eiken Chemical Co.) to more than 20,000 individuals who were randomly selected from municipal databases. Along with the tests, the researchers sent informational material, consent forms, and postage-paid return envelopes.

Significantly more subjects returned the I-FOBTs (59.6 per cent, versus a return rate of 46.9 per cent for the G-FOBTs). Altogether, nearly 11,000 tests were returned.

Positive tests were more common with I-FOBT (5.5 per cent) than with G-FOBT (2.4 per cent), the authors report. I-FOBT also detected more cancers (24) and advanced adenomas (121) than G-FOBT (11 cancers and 46 advanced adenomas).

Positive predictive values, however, did not differ significantly between I-FOBT and G-FOBT, the investigators say, but the estimated specificity of I-FOBT was slightly lower than that of G-FOBT for both cancer and advanced adenomas.

"At least theoretically, a five-year screening strategy with the I-FOBT would approach the sensitivity of colonoscopy screening every five years," Dr. van Rossum said. "So besides the fact that I-FOBT performs better than G-FOBT, we should also consider that it performs better than primary colonoscopy screening."

"At the moment we are completing a study about the cost-effectiveness of I-FOBT versus G-FOBT or no screening," Dr. van Rossum added. "Both G-FOBT and I-FOBT proved to be cost-effective in several studies in the past, but we are in the unique opportunity to perform a cost-effectiveness study based on empirical data comparing the two types of FOBT."

Gastroenterology 2008;135:82-90