The goal of this longitudinal observational study is to learn if impulse oscillometry (IOS) has an added value in asthma monitoring in adult asthma patients who are prescribed a change in asthma maintenance therapy. The main questions it aims to answer are: * Is there a difference in the change in IOS parameters and FEV1 respectively, stratified according to change in asthma control test? * Is there a difference in the change in IOS parameters and FEV1 respectively, stratified according to change in other questionnaire such as the asthma control questionnaire and the asthma quality of life questionnaire. * Are the proposed minimal clinically important differences (MCIDs) valid for short follow-up periods (3 - 6 months)? Participants will undergo lung function testing (full lung function, multiple breath nitrogen washout, impulse oscillometry) and questionnaires (asthma control test, asthma control questionnaire, asthma quality of life questionnaire), once during the baseline visit and once during the follow-up visit three to six months later.