Impact of between-hospital volume and within-hospital volume on mortality and readmission rates for trauma patients in California.

OBJECTIVE: Previous research assessing the impact of between-hospital trauma volume (high volume centers vs. low volume centers) and outcomes has been inconsistent. Furthermore, previous research has not considered temporal variations in within-hospital volume (a center having higher than average volume vs. lower than average volume) as a covariate. The objective of this study was to determine the relationship of between-hospital and within-hospital trauma volume and two measures of hospital quality of care.

DESIGN: Multivariable, hierarchical, mixed effects, logistic regression analyses of a population-based nonconcurrent cohort from 1995 to 1999.

SETTING: Thirty-nine nonfederal California hospitals included in the California Patient Discharge Data Set designated by local Emergency Medical Services authorities as adult trauma centers.

PATIENTS: All nonelderly adult trauma patients, 16-64 yrs (n = 54,352), and elderly adult trauma patients, >65 yrs (n = 47,656), admitted with an Injury Severity Score >9.

INTERVENTIONS: None.

MEASUREMENTS AND MAIN RESULTS: Severity adjusted in-hospital mortality rate and 30-day trauma-related readmissions were analyzed. Among nonelderly adult patients, higher annual between-hospital trauma volume was not associated with mortality rate (odds ratio, 1.02 for each 100 admissions; 95% confidence interval, 0.99, 1.06) but was associated with higher risk of readmission (odds ratio, 1.19 for each 100 admissions; 95% confidence interval, 1.13, 1.26). Among elderly adult patients, higher annual between-hospital trauma volume was associated with lower mortality (odds ratio, 0.79 for each 100 admissions; 95% confidence interval, 0.71, 0.87) but was not associated with risk of readmission (odds ratio, 0.96 for each 100 admissions; 95% confidence interval, 0.90, 1.04). Higher than average monthly within-hospital trauma volume was associated with higher odds of readmission (odds ratio, 1.11 for a volume deviation of ten patients per month; 95% confidence interval, 1.01, 1.21) among elderly adult patients.

CONCLUSIONS: The findings of this study in the context of previous research suggest that relationships between trauma volume and outcomes exist but depend on which patient populations are studied and how the data are analyzed. Furthermore, trauma centers may be subject to the detrimental effects of high temporal volume overextending existing services and capacity. Since this study found that both between-hospital volume and within-hospital volume measures are associated with outcomes, we recommend that both measures be included in future volume-outcome investigations.