RT Journal Article SR Electronic T1 Missing sputum samples are common in asthma intervention studies and successful collection at follow-up is related to improvement in clinical outcomes JF ERJ Open Research JO erjor FD European Respiratory Society SP 00612-2021 DO 10.1183/23120541.00612-2021 A1 Laurits Frøssing A1 Morten Hvidtfeldt A1 Alexander Silberbrandt A1 Asger Sverrild A1 Celeste Porsbjerg YR 2022 UL http://openres.ersjournals.com/content/early/2022/01/06/23120541.00612-2021.abstract AB