[1] Ghent Univ Hosp, Dept Resp Med, B-9000 Ghent, Belgium
[2] Erasmus MC, Dept Epidemiol, NL-3000 CA Rotterdam, Netherlands
[3] Erasmus MC, Dept Resp Med, NL-3000 CA Rotterdam, Netherlands
[4] Univ Oxford, Nuffield Dept Med, Resp Med Unit, Oxford, England
[5] Univ Oxford, Nuffield Dept Med, Oxford Resp BRC, Oxford, England