Statistical analysis
In order to comparatively assess the effect of center variations, the induction therapy utilization rate was stratified into equal terciles, which were classified as low (<29%), intermediate (29-66%) or high (>67%) induction therapy utilization. The data is described as proportions (%) for categorical variables and as means or medians for continuous variables, with standard deviation (SD) or interquartile range (IQR), respectively. Pearson’s Chi2 test was used to compare categorical variables and one-way analysis of variance (ANOVA) or students t-tests were used to compare continuous data. All statistical tests were 2-sided, and p<0.05 was considered significant. Multilevel mixed-effects logistic regression models were used to estimate the relationship of induction therapy and post-OHT clinical outcomes accounting for inter-center variability. The primary endpoint was drug-treated acute rejection within the first year following OHT. Model A included only the individual transplant centers as random effect parameters. Model B added the recipient and donor risk factors identified as fixed effect parameters to Model A. Finally, Model C, added induction therapy variable as a fixed effect parameter to Model B. Changes in the estimate of the between-centers variance may suggest the distribution of variables across centers. The Variance Partition Coefficient (VPC) was estimated to evaluate the proportion of variation attributable to systematic differences between centers characteristics in the context of a binary dependent variable. This contribution as a percentage was calculated as [X /X+3.29] * 100, where X is the variance estimate from the random effect component of the model (15). Risk-adjusted Cox proportional hazards models were used to evaluate the adjusted impacts of induction therapy on mortality. Kaplan-Meier estimates were used to evaluate the longitudinal impacts of the use of induction therapy on survival. Pearson’s correlation coefficient (R) was utilized to assess the correlation between pre-OHT risk scores and the use of induction therapy. All analyses were performed with version 15 STATA statistical software (StataCorp LP, College Station, TX).