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Contributors DF conceived this study and provided financial support. DF and JL designed the study, prepared and revised the manuscript; also had key roles in the study. JH, LT, LC and YM took part in the design of the study and in sample collection. JL, LT, LC and DF conducted data management. JL conducted data follow-up. LC and LT undertook data checking. JH, JL and DF undertook statistical analysis. DF was responsible for project management.
Funding This work was supported by the National Natural Science Foundation of China (81030019).
Competing interests None declared.
Patient consent Obtained.
Ethics approval Ethics Committee at Peking University Third Hospital.
Provenance and peer review Not commissioned; externally peer reviewed.