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Investigation of associations of the diagnostic diversity and volumes with efficiency and quality of care.Prospective observational study.Thirty-two pediatric intensive care units (PICUs), 16 selected by random cluster sampling, and 16 volunteering.Consecutive admissions of 11,165 patients.The main outcome measures were length of PICU stay (LOS) and mortality rate, adjusted by generalized linear regression and multivariate logistic regression, respectively. Each diagnosis was categorized into 21 predefined, mutually exclusive categories. Diagnostic diversity of each PICU was characterized by an information-theoretical measure (entropy). For a patient-level analysis, the associations of this measure and PICU patient volume with outcomes were using regression models. For an institution-level analysis, the outcome measures of each PICU were adjusted using ratios of observed/predicted (by the regression models) values, and the associations of these ratios with diagnostic diversity and patient volume were investigated using linear bivariate regressions.Diagnostic diversity ranged in the PICUs from 0.823 to 0.928, when standardized to the uniform distribution with entropy of 1. Congenital heart diseases (12.6%) head traumas (11.5%), other central nervous system conditions (9.7%), and pneumonias (8.7%) constituted the largest diagnostic categories. Patient-level analysis indicated that longer adjusted LOS was associated with larger diagnostic diversity (p < .0001) and lower admission volumes (p < .0001). However, for a given increase in diagnostic diversity, a large LOS increase was associated with low-volume, but not high-volume units. Severity-adjusted mortality rates were inversely related (p = .036) only with admission volumes, but not diagnostic mix. Institution-level standardized LOS ratios correlated with diagnostic diversity (r2 = 0.145;p = .031). Institution-level standardized mortality ratios were inversely related (r2 = 0.123;p = .049) with admission volumes.Patient volumes encountered in a PICU are important for maintaining quality and efficiency of care. In low-volume units, fewer diagnoses and higher volumes were both associated with higher efficiencies. In high volume units, diagnosis-specific volumes were generally large enough for achieving diagnosis-independent efficiency. Diagnostic mix was not associated with PICU mortality ratios, but higher PICU volumes were associated with lower mortality rates.