Benefits of screening include a modest reduction in colorectal cancer mortality, a possible reduction in cancer incidence through the detection and removal of colorectal adenomas, and potentially, the less invasive surgery that earlier treatment of colorectal cancers may involve.Harmful effects of screening include the psycho-social consequences of receiving a false-positive result, the potentially significant complications of colonoscopy or a false-negative result, the possibility of overdiagnosis (leading to unnecessary investigations or treatment) and the complications associated with treatment.
Colorectal cancer mortality: effectiveness of biennial screening for fecal occult blood.
In 1993, a randomized controlled trial in Minnesota showed, after 13 years of follow-up, that annual fecal occult blood testing was effective in reducing colorectal cancer mortality by at least 33%. Biennial screening (i.e., every 2 years) resulted in only a 6% mortality reduction. Two European trials (in England and in Denmark) subsequently showed statistically significant 15% and 18% mortality reductions with biennial screening. Herein, we provide updated results-through 18 years of follow-up--from the Minnesota trial that address the apparent inconsistent findings among the trials regarding biennial screening.
                From 1976 through 1977, a total of 46551 study subjects, aged 50-80 years, were recruited and randomly assigned to an annual screen, a biennial screen, or a control group. A screen consisted of six guaiac-impregnated fecal occult blood tests (Hemoccult) prepared in pairs from each of three consecutive fecal samples. Participants with at least one of the six tests that were positive were invited for a diagnostic examination that included colonoscopy. All participants were followed annually to ascertain incident colorectal cancers and deaths.
                The numbers of deaths from all causes were similar among the three study groups. Cumulative 18-year colorectal cancer mortality was 33% lower in the annual group than in the control group (rate ratio, 0.67; 95% confidence interval [CI] = 0.51-0.83). The biennial group had a 21% lower colorectal cancer mortality rate than the control group (rate ratio, 0.79; 95% CI = 0.62-0.97). A marked reduction was also noted in the incidence of Dukes' stage D cancers in both screened groups in comparison with the control group.
                The results from this study, together with the other two published randomized trials of fecal occult blood screening, are consistent in demonstrating a substantial, statistically significant reduction in colorectal cancer mortality from biennial screening.