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.