Re: Effectiveness of flexible sigmoidoscopy screening in men and women and different age groups: pooled analysis of randomised trials
We thank Drs Braillon, Doherty and Hawkins for their response to our article.
First, our paper is a meta-analysis, which is why we adhered to the PRISMA guidelines (1). The CONSORT guidelines, as mentioned by Dr Doherty, are for individual randomized trials, and these guidelines do not apply to our paper.
The Cochrane collaboration calls for caution when calculating absolute risk reductions from meta-analyses (2) and state that the reporting of relative risk should be preferred. Absolute risk reductions computed from aggregated data ignore the randomization within trials and may provide seriously misleading results. Relative risks (e,g, risk ratios or odds ratios) are less sensitive to differences in baseline risk in the various control groups. To exemplify this, consider the rectosigmoid cancer incidence for males aged 55-59 years and 70-74 years in the PLCO study (Figure 2 in the paper). The risk ratios are almost identical, 0.63 and 0.64, respectively. However, because the cancer risk increases with age, the baseline risks (in the control groups) are very different (0.007 and 0.012, respectively), and accordingly, the risk differences also vary (-0.0026 versus -0.0041, respectively). The number needed to treat can be calculated as the inverse of (absoulte value of ) the risk difference, 384 and 244, respectively. The Cochrane handbook (available online at http://handbook.cochrane.org) provides equations for obtaining absolute risks from meta-analyses and their corresponding 95% confidence intervals, but we did not calculate them due to the considerations mentioned above.
Dr Braillon asks how women can benefit from screening even if their risk is much lower than for men. As we have shown in the previous section, relative risk reductions are independent of the baseline risk, unless the baseline risk is zero.
We strongly disagree with Dr Braillon, stating that the best screening program is the program that people adhere to with high fidelity. We show that flexible sigmoidoscopy may be inappropriate in older women (3). In the 30-year follow up report of the Minnesota-trial, biennial screening for faecal occult blood (guaiac based test) reduced colorectal cancer mortality in men, but not in women (4). Screening for colorectal cancer does not come without a risk of harm. If we recommend screening tests with no or minimal benefit, only the risk of harm and the costs remain.
1. Moher D, Liberati A, Tetzlaff J, Altman DG, Group P. Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. PLoS Med. 2009;6(7):e1000097.
2. Higgins JP, Green S, editors. Cochrane Handbook for Systematic Reviews of Interventions. Version 5.1.0 [updated March 2011]: The Cochrane Collaboration, 2011. Available from http://www.cochrane-handbook.org.
3. Holme O, Schoen RE, Senore C, Segnan N, Hoff G, Loberg M, et al. Effectiveness of flexible sigmoidoscopy screening in men and women and different age groups: pooled analysis of randomised trials. BMJ. 2017;356:i6673.
4. Shaukat A, Mongin SJ, Geisser MS, Lederle FA, Bond JH, Mandel JS, et al. Long-term mortality after screening for colorectal cancer. N Engl J Med. 2013;369(12):1106-14.
Competing interests: No competing interests