title: Asthma symptom utility index: reliability, validity, responsiveness, and the minimal important difference in adult asthmatic patients.
abstract: 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.
  
  
  OBJECTIVE
  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.
authors.affiliation: Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Christian
authors.initials: C
authors.lastname: Bime
authors.forename: Christine Y
authors.initials: CY
authors.lastname: Wei
authors.forename: Janet T
authors.initials: JT
authors.lastname: Holbrook
authors.forename: Marianna M
authors.initials: MM
authors.lastname: Sockrider
authors.forename: Dennis A
authors.initials: DA
authors.lastname: Revicki
authors.forename: Robert A
authors.initials: RA
authors.lastname: Wise
mesh_terms: D000328:Adult; D001249:Asthma / Q000175:diagnosis* / Q000453:epidemiology* / Q000523:psychology; D018450:Disease Progression; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D015337:Multicenter Studies as Topic; D011379:Prognosis; D011594:Psychometrics / Q000379:methods; D016032:Randomized Controlled Trials as Topic; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D013147:Spirometry / Q000706:statistics & numerical data; D011795:Surveys and Questionnaires