Statistical analyses
Statistical analyses were performed using STATA 11 statistical software (StataCorp LP College Station, TX). Continuous variables were expressed as mean ± standard deviation (SD) and were compared between groups by independent t tests. Variables that were non-normally distributed as determined by the Shapiro-Wilk test of normality were reported as the median and interquartile range (IQR, i.e., the arithmetic difference between the 25th percentile value and the 75th percentile value). Categorical variables with ≥ three groups were compared with one-way analysis of variance (ANOVA). Up to three covariates were adjusted for in multiple regression analyses: age, gender, and presence of diabetes mellitus. A 2-tailed p value of <0.05 was considered statistically significant. When ANOVA was not used, no statistical adjustment was made for the number of comparisons performed.