[Definition of low threshold volumes for quality assurance: conceptual and methodological issues involved in the definition and evaluation of thresholds for volume outcome relations in clinical care].

In a large number of mostly retrospective association studies, a statistical relationship between volume and quality of health care has been reported. However, the relevance of these results is frequently limited by methodological shortcomings. In this article, criteria for the evidence and definition of thresholds for volume-outcome relations are proposed, e.g. the specification of relevant outcomes for quality indicators, analysis of volume as a continuous variable with an adequate case-mix and risk adjustment, accounting for cluster effects and considering mathematical models for the derivation of cut-off values. Moreover, volume thresholds are regarded as surrogate parameters for the indirect classification of the quality of care, whose diagnostic validity and effectiveness in improving health care quality need to be evaluated in prospective studies.