Statistical analyses

Statistical analysis was performed using the Statistical Package for Social Sciences Software (SPSS 21.0 for Windows, SPSS Inc., Chicago, IL, USA) and Medcalc 9 (Acacialaan 22, B-8400 Ostend, Belgium). The normal distribution of variables was evaluated with Shapiro-Wilk test. Parameters with normal distribution were assessed with parametric methods, whereas nonparametric methods were used for testing variables without normal distribution. Two independent groups are compared via Independent-Samples t-test and Mann-Whitney U test. Categorical variables are analyzed by Pearson Chi-Square test. Performance regarding variability of classification was tested by sensitivity, specificity, positive predictivity, negative predictivity and receiver operating curve (ROC) analysis. Data are displayed as mean and standard deviation or as median and interquartile range. The level of statistical significance was set at P<0.05.