Statistical analysis
SAS 9.13 (SAS Institute, NC, USA) was used for database management and statistical analyses. Continuous variables were expressed as the mean and standard deviation (SD) or median and interquartile range (IQR) where appropriate. The t test was used to analyze normally distributed continuous variables, whereas the Mann–Whitney U test was used to analyze nonnormally distributed continuous variables. Categorical variables were reported as frequencies and percentages, and analyzed using the chi-square test. P values less than 0.05 were considered statistically significant.