 The study will enrol patients within GINA classification 4 and 5 who are known to difficult asthma services across four UK sites. Some patients will recently have been commenced on Mepolizumab, whilst others will be commenced on the drug on entry to the study. The patients will have baseline measurements of biomarkers, lung function, sputum analysis and quality of life questionnaires on study entry, after which patients will be asked to contact the clinic at the first signs of worsening asthma symptoms to arrange a clinic visit prior to commencing rescue treatment. They will be clinically assessed with review of peak flow and symptom diaries, measurements taken at baseline will be repeated and a decision on the nature of the exacerbation and treatment required will be made. 