Statistical analysis

SPSS 19.0 statistical software (SPSS Chicago, Ilinois, USA) was used for data analysis. Data are expressed as frequencies and percentages, with means and standard deviations as appropriate. Chi-square tests, simple t-tests and Fisher exact tests were used to preliminarily screen predictors (p < 0.1), and then all the candidate factors were further confirmed by multivariate logistic regression analysis (p < 0.05). Independent simple t-tests, chi-square tests and Fisher exact tests were used to verify these predictors (p < 0.05). For quantitative data, we analysed the receiver operating characteristic (ROC) curve to determine the cut-off value of the factor. Statistical significance was established at p < 0.05.