CRC screening with flexible sigmoidoscopy reduces risk for death

The USPSTF conclusion was partially based on a meta-analysis of four randomized trials that compared flexible sigmoidoscopy screening with no screening. The meta-analysis aggregated results from the two age cohorts of the NORCCAP (Norwegian Colorectal Cancer Prevention) trial as if the cohorts were a single trial. This analysis created a Simpson paradox that obscured the reduction in all-cause mortality by changing two statistically nonsignificant reductions into a statistically significant increase. The effect was large enough to nullify the reductions in all-cause mortality of the other trials in the meta-analysis.

Researchers assessed results of the NORCCAP study for this Simpson paradox and repeated meta-analysis of all-cause mortality outcomes for screening flexible sigmoidoscopy using the 2 NORCCAP age cohorts as individual trials. They found that looking at the cohorts as two separate groups, rather than aggregating them, the relative risk for all-cause mortality favored screening with flexible sigmoidoscopy. The authors conclude that if the goal of screening is to reduce the risk for death, then the evidence supporting flexible sigmoidoscopy is substantially stronger than that of other screening methods.