['Number needed to screen': a tool for assessment of prevention programs]

The number needed to screen (NNS) is defined as the number of people that need to be screened for a given period to prevent one death or adverse event. The NNS is one of the criteria with which the efficacy of a screening programme can be determined. This article introduces the NNS to Dutch literature. The NNS may be calculated with mortality figures and the absolute risk reduction obtained by a prevention programme. Another method is to multiply the number needed to treat (NNT) with the number of people that have to be screened in order to find one patient with the disease. Based on published figures, the NNS's per year to prevent one death in current screening programmes within general practice in the Netherlands are approximately 2560 for cervical cancer screening, about 1000 for breast cancer screening, and circa 2340 for the detection of hypertension in patients aged 55-75 years.