The number remaining at risk: an adjunct to the number needed to treat.

Although the number of patients needed to treat (NNT) to prevent an adverse clinical event is of great clinical value to practising physicians, it is limited in that it fails to provide a measure of prognosis among patients not achieving benefit. For example, if the NNT is 100, what is likely to happen to the other 99? The number remaining at risk (NRR), which is an index that enhances the value of the NNT, is described. The NRR is the ratio of the residual event rate among treated patients and the absolute reduction in outcome events (NRR = experimental event rate [EER]/control event rate [CER]-EER), where EER and CER are the event rates among experimental and control groups, respectively. This index represents the number of events likely to occur among the NNT, or the odds of experiencing an adverse outcome event as opposed to deriving benefit from therapy. The NRR is a simple index that can easily be calculated from the published results of a clinical trial. As an adjunct to the NNT, it provides a measure of the impact of therapy and the average prognosis of remaining patients.