Asthma and lower airway diseaseThe minimally important difference of the Asthma Control Test
Section snippets
Patients
Analyses to determine the MID for the ACT were conducted by using data from each of 4 independent study samples. The patient characteristics, the conditions under which data were collected, and the types of data measured varied across the 4 samples. Details on the study design and each of the 4 samples have been previously published elsewhere. Briefly, sample 1 consisted of 1090 prescreened asthmatic patients (mean age, 47.8 ± 14.0 years; 61% female) from a large nationally representative
Distribution-based results
Results of the distribution-based analyses are presented in Table II. Examination of results indicates a good deal of stability in both the reliability and MID estimates of ACT scores across all 4 samples. Both the 0.5 SD and 1 SEM criteria produced similar results, with the former mostly yielding estimates slightly greater than 2 (range, 2.09–2.45; M = 2.21) and the latter mostly yielding estimates slightly less than 2 (range, 1.77–2.05; M = 1.88). Estimates produced by the 2 SEM criteria
Discussion
Asthma control is the goal of asthma therapy,2, 3 and the ACT has been validated in several settings and with multiple criterion measures to reflect asthma control.4, 7, 8, 9 However, the MID for the instrument must be defined to optimally use the ACT in intervention studies or to follow individual patents over time. The current study used both distributional and anchor-based methods in multiple populations to identify the MID of the ACT. Almost all (35/39) of the individual results (Tables II
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Disclosure of potential conflict of interest: M Schatz has received research support from GlaxoSmithKline, Merck, Aerocrine, and Genentech. M. Kosinski is a consultant for GlaxoSmithKline. M. E. Watson and P. Jhingran are employees of GlaxoSmithKline. The rest of the authors have declared that they have no conflict of interest.
Supported by an unrestricted grant from GlaxoSmithKline.