1. Department of Pulmonary and Critical Care Medicine, Xijing Hospital, Fourth Military Medical University, Xi'an, Shaanxi, P.R. China
2. Department of Medical Statistics, Fourth Military Medical University, Xi'an, Shaanxi, P.R. China
3. Department of Respiratory Medicine, Shaanxi Provincial Second People's Hospital, Xi'an, Shaanxi, P.R. China
4. Department of Pathology, Fourth Military Medical University, Xi'an, Shaanxi, P.R. China