1 Department of Pulmonary Disease and Groningen Research Institute for Asthma and COPD, University of Groningen, University Medical Center Groningen, Groningen 9713GZ, Netherlands. Electronic address: t.m.kole@umcg.nl.
2 Department of Pulmonary Disease and Groningen Research Institute for Asthma and COPD, University of Groningen, University Medical Center Groningen, Groningen 9713GZ, Netherlands.