[Volume standards: quality through quantity? relationship between treatment volume and outcomes not well founded]

Three domains are accepted as indicators that make the quality of hospital care quantifiable and transparent: process, structure and outcome. In recent years volume thresholds for surgical interventions have been added as new quality indicators, rendering the assumption that more volume leads to better outcomes. This assumption has been established for many procedures in peer-reviewed literature since the 1970s. Although there is consensus about the fact that volume is a proxy measure for other factors influencing the outcome of care, available research does not shed much light on the mechanisms underlying the volume-outcome relationship. Moreover, the methodological quality of the available research has several limitations. Despite the incomplete substantiation, volume thresholds are now embedded in 'risk-based' supervision by the health inspectorate as well as purchasing policies by health insurers.