Statistical Analysis
All statistical analyses were performed with SPSS v.23.0 (IBM Corp., Armonk, NY, USA). Associations between nominal variables were examined using χ2 test and Fisher’s exact test. Associations between continuous and quantitative variables were examined using Mann-Whitney U-Test. Potential confounding variables were assessed by Cox proportional hazards regression model. A two-sided p-value of <.05 was considered statistically significant for all analyses.