Screening and the number needed to treat

Objective-To develop a measure analogous to the "number needed to treat" (NNT), which would be appropriate for population based screening programmes. Methods-A previous measure analogous to the NNT, the "number needed to screen" (NNS), calculated as the reciprocal of the absolute risk reduction, does not account for participation and selection effects in screening trials. A new measure is proposed; the NNBS (number needed to be screened) derived from the NNT adjusted for participation in screening and for selection effects associated with participation in screening. The NNBS for breast cancer screening and colorectal cancer screening are calculated, based on the results of randomised controlled trials. Results-In the first 8 years from the start of each screening programme, the NNS to prevent one death from breast cancer was 781 women (95% confidence interval (95% CI) 550 to 1560). The NNS to prevent one death from colorectal cancer was 1250 people (95% CI 690 to 9090). The NNBS was 23% lower than the NNS for breast cancer screening (NNBS=601), and 45% lower than the NNS for colorectal cancer (NNBS=688). Conclusions-The NNT calculations may make population based screening programmes seem expensive and inefficient compared with other interventions. A new measure, the number needed to be screened (NNBS), which takes into account the participation rate adjusted for selection, may be more appropriate, especially when comparing screening programmes with disparate participation rates, or when comparing screening with other interventions.