Received: 17 May 2018
Accepted: 30 November 2018
First Online: 13 December 2018
Ethics approval and consent to participate
: Written informed consent was obtained from all included subjects, which was approved by the institutional review board and ethics committee of the Qingdao University Medical School, Qingdao, China.
: Not applicable.
: The authors declare that they have no competing interests.
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