Methods
We used the Pediatric Health Information System (PHIS), an electronic database of children’s hospitals in the US. The study was deemed exempt by the local Institutional Review Board. De-identified patients under the age of 21 who underwent HSCT at one of the 42 PHIS hospitals from 2000-2012 were analyzed using data abstracted with ICD-9 codes. All statistical analyses were performed using SAS software, version 9.3 (SAS Institute, Cary, NC) or the base R statistical package (R Foundation for Statistical Computing, Vienna, Austria). All calculated P values were 2-sided, and P < .05 were considered statistically significant.