Asthma symptom utility index: reliability, validity, responsiveness, and the minimal important difference in adult asthmatic patients.

BACKGROUND: The evaluation of asthma symptoms is a core outcome measure in asthma clinical research. The Asthma Symptom Utility Index (ASUI) was developed to assess the frequency and severity of asthma symptoms. The psychometric properties of the ASUI are not well characterized, and a minimal important difference (MID) is not established. OBJECTIVES: We assessed the reliability, validity, and responsiveness to change of the ASUI in a population of adult asthmatic patients. We also sought to determine the MID for the ASUI. METHODS: Adult asthmatic patients (n = 1648) from 2 previously completed multicenter randomized trials were included. Demographic information, spirometric results, ASUI scores, and other asthma questionnaire scores were obtained at baseline and during follow-up visits. Participants also kept a daily asthma diary. RESULTS: The internal consistency reliability of the ASUI was 0.74 (Cronbach α). Test-retest reliability was 0.76 (intraclass correlation). Construct validity was demonstrated by significant correlations between ASUI scores and Asthma Control Questionnaire scores (Spearman correlation r = -0.79; 95% CI, -0.85 to -0.75; P < .001) and Mini Asthma Quality of Life Questionnaire scores (r = 0.59; 95% CI, 0.51-0.61; P < .001). Responsiveness to change was demonstrated, with significant differences between mean changes in ASUI scores across groups of participants differing by 10% in percent predicted FEV(1) (P < .001) and by 0.5 points in Asthma Control Questionnaire scores (P < .001). Anchor-based and statistical methods support an MID for the ASUI of 0.09 points. CONCLUSIONS: The ASUI is reliable, valid, and responsive to changes in asthma control over time. The MID of the ASUI (range of scores, 0-1) is 0.09.