Evaluating minimal important differences and responder definitions for the asthma symptom diary in patients with moderate to severe asthma

BACKGROUND: The Asthma Symptom Diary was developed to assess severity of symptoms in patients with moderate to severe asthma, and has evidence supporting reliability and validity. Only limited information is available on sensitivity to change and responder definitions for the Asthma Symptom Diary. OBJECTIVES: Main study objectives were to evaluate sensitivity to change and provide responder definitions for clinically meaningful effects for the Asthma Symptom Diary. METHODS: This is a secondary analysis of Phase II clinical trial data in patients with moderate to severe asthma, Asthma Symptom Diary (ASD) was collected daily during the 24-week study. The Asthma Control Questionnaire and the Patient Global Assessment were collected at baseline, and week 12 and 24. Analysis of covariance (ANCOVA) models were used to evaluate sensitivity to change in Asthma Symptom Diary scores after 12 and 24 weeks of treatment. Anchor-based methods, using Asthma Control Questionnaire and Patient Global Assessment defined anchors, were used to identify minimal important differences and various responder criteria for changes in mean 7-day ASD score, symptomatic days, and minimal symptom days. RESULTS: Sample was 59% female, 81% White, with a mean age of 47.3 (SD = 13.6) years. ANCOVAs demonstrated significant differences in baseline to week 12 and week 24 changes in mean 7-day Asthma Symptom Diary scores and symptomatic days by Asthma Control Questionnaire (all p < 0.001) and Patient Global Assessment anchors (all p < 0.001). Meaningful responders, from the patient’s perspective, were defined as improvements of 0.5–0.6 points (SD = 0.6; scale range 0 to 4) in mean 7-day Asthma Symptom Diary scores, and as a reduction of 2 to 3 Asthma Symptom Diary-based symptomatic days. CONCLUSION: The Asthma Symptom Diary was responsive to changes in clinical status in patients with moderate to severe asthma. Responder definitions were identified, including symptomatic days, for evaluating individual level treatment effects in clinical trials.