1. Division of Pulmonary & Critical Care Medicine, Department of Internal Medicine, Cheongju St. Mary Hospital, Cheongju, Korea.
2. Division of Pulmonary & Critical Care Medicine, Department of Internal Medicine, University of Ulsan College of Medicine, Asan Medical Center, Seoul, Korea.
3. Department of Laboratory Medicine, University of Ulsan College of Medicine, Asan Medical Center, Seoul, Korea.