Why do high-volume hospitals achieve better outcomes? A systematic review about intermediate factors in volume-outcome relationships

Abstract Objective To assess the role of process and structural factors in volume–outcome relationships. Data sources Pubmed electronic database, until March 2014. Study design Systematic review. Based on a conceptual framework, peer-reviewed publications were included that presented evidence about explanatory factors in volume–outcome associations. Data collection Two reviewers extracted information about study design, study population, volume and outcome measures, as well as explanatory factors. Included publications were appraised for methodological quality. Principal findings After screening 1756 titles, 27 met our inclusion criteria. Three main categories of explanatory factors could be identified: 1. Compliance to evidence based processes of care ( n  = 7). 2. Level of specialization ( n  = 11). 3. Hospital level factors ( n  = 10). In ten studies, process and/or structural characteristics partly explained the established volume–outcome association. The median quality score of the 27 studies was 8 out of a possible 18 points. Conclusions The vast majority of volume–outcome studies do not focus on the underlying mechanism by including process and structural characteristics as explanatory factors in their analysis. The methodological quality of studies is also modest, which makes us question the available evidence for current policies to concentrate care on the basis of volume.