Statistical analysis
Continuous variables are presented as mean (standard deviation), and categorical variables are presented as numbers (percentages). All statistical analyses were performed using the SPSS version 21.0 software (SPSS Inc., Chicago, IL, USA). Student’s t-test was used to compare continuous variables, and the chi-squared test or Fisher’s exact test was used to analyze categorical variables. Statistical significance was set at a two-sided p value < 0.05.