According to the United States Preventive Services Task Force (USPSTF) guidelines for colorectal cancer screening, no colorectal cancer screening methods reduced the all-cause mortality. However, a new study published in Annals of Internal Medicine reported that a re-analysis of all-cause mortality in the USPSTF evidence showed that colorectal cancer screening with flexible sigmoidoscopy reduced the risk of death.
Andrew W. Swartz, MD, from the Yukon-Kuskokwim Delta Regional Hospital and colleagues noted that the USPSTF conclusion partly depended on a meta-analysis of four randomized trials which compared flexible sigmoidoscopy screening with no screening. The meta-analysis collected results from the two age groups of the NORCCAP (Norwegian Colorectal Cancer Prevention) trial as if the groups were a single trial.
In the NORCCAP study, two different trial groups (50–54 years group and 55–64 years group) were included since a post-screening decided to expand the inclusion age to younger people and the groups were randomly assigned separately. The younger group (50 to 54 years) had a lower event rate and was randomly allotted with a screen–control ratio of 1:5.4 instead of the ratio of 1:3 used in the older group.
The meta-analysis produced a Simpson paradox that concealed the reduction in all-cause mortality by altering two statistically non-significant 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.
The scientists assessed the NORCCAP study’s outcomes to create Simpson paradox. The researchers used the 2 NORCCAP age groups as individual trials and repeated the meta-analysis of all-cause mortality results for screening flexible sigmoidoscopy. The research team found that the relative risk for all-cause mortality favored screening with flexible sigmoidoscopy when two groups were considered as separate groups, instead of combining them.
The screening aimed to decrease the risk of mortality. Thus, the evidence supporting flexible sigmoidoscopy was considerably stronger when compared to other screening methods, concluded by the authors.
Flexible sigmoidoscopy reduces risk for death; this result represents a milestone in colorectal cancer screening. These findings suggest that the USPSTF guidelines for colorectal cancer screening, which concluded that no colorectal cancer screening methods reduced all-cause mortality, could be reassessed.