Indirect Standardization as a Case Mix Adjustment Method to Improve Comparison of Children’s Hospitals’ Antimicrobial Use

Abstract Antimicrobial use (AU) in days of therapy per 1000 patient-days (DOT/1000 pd) varies widely among children’s hospitals. We evaluated indirect standardization to adjust AU for case mix, a source of variation inadequately addressed by current measurements. Hospitalizations from the Pediatric Health Information System were grouped into 85 clinical strata. Observed to expected (O:E) ratios were calculated by indirect standardization and compared with DOT/1000 pd. Outliers were defined by O:E z-scores. Antibacterial DOT/1000 pd ranged from 345 to 776 (2.2-fold variation; interquartile range [IQR] 552–679), whereas O:E ratios ranged from 0.8 to 1.14 (1.4-fold variation; IQR 0.93–1.05). O:E ratios were moderately correlated with DOT/1000 pd (correlation estimate 0.44; 95% confidence interval, 0.19–0.64; P = .0009). Using indirect standardization to adjust for case mix reduces apparent AU variation and may enhance stewardship efforts by providing adjusted comparisons to inform interventions. The indirect standardization method can be used to adjust children’s hospitals’ antimicrobial use data for differences in case mix and generate observed to expected ratios for comparison. Case mix adjustment reduces apparent variation between hospitals and reveals unexpected outlier hospitals.