Statistical analysis was performed using SPSS 13.0 statistical software (SPSS Inc., Chicago, USA). Data were expressed as the mean ± standard deviation (SD). The data were analyzed with the Shapiro-Wilk test and the Levene statistic for normality and homogeneity of variance, respectively. The difference between two quantitative groups was compared with an independent-sample t-test or the Mann-Whitney U-test as appropriate, and correlations were analyzed with the Pearson or Spearman correlation test. The chi-square test or Fisher's exact test was used to compare the proportions of the two groups. All tests were two-tailed with the risk set at 5%, and the statistical significance was set as p < 0.05.