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1 Dept of Clinical Epidemiology and Biostatistics, McMaster University Faculty of Health Sciences, Hamilton, Ontario, Canada, 2 GlaxoWellcome Inc., Research Triangle Park, North Carolina, USA, 3 Dept of Epidemiology and Biostatistics, University of Western Ontario, London, Ontario, Canada
CORRESPONDENCE: E. Juniper, 20 Marcuse Fields, Bosham, West Sussex, PO18 8NA, UK. Fax: 44 1243573680
Keywords: asthma, measurement, quality of life, questionnaires, utilities
Received: October 9, 2000
Accepted March 9, 2001
This study was supported through a grant from GlaxoWellcome.
With interest in health economics growing, there is a demand for valid methods for measuring health-related quality of life (HRQL) in asthma using utilities. The aims of this study were to develop disease-specific versions of the standard gamble and rating scale, to compare their measurement properties with those of the Asthma Quality of Life Questionnaire (AQLQ) and the Medical Outcomes Survey Short-Form 36 (SF-36), as well as to determine their validity for assessing asthma-specific quality of life.
Forty adults with symptomatic asthma participated in a 9-week observational study. Participants completed the standard gamble, rating scale, AQLQ, SF-36 and other measures of clinical asthma status at baseline and after 1, 5 and 9 weeks.
In patients whose asthma was stable between assessments, reliability was good for the rating scale (intraclass correlation coefficient (ICC)=0.89) and the AQLQ (ICC=0.95) but more modest for the SF-36 mental score (ICC=0.68), SF-36 physical score (ICC= 0.65) and standard gamble (ICC=0.59). The responsiveness index was highest in the AQLQ (1.35), followed by the rating scale (0.74), the physical score of the SF-36 (0.61) and the standard gamble (0.31). Construct validity (correlation with other indices of health status) was strongest for the AQLQ and the rating scale.
In conclusion, both the disease-specific rating scale and the Asthma Quality of Life Questionnaire have strong measurement properties for measuring asthma-specific quality of life; the Short-Form 36 health survey physical summary score has more modest properties. Although the disease-specific standard gamble has acceptable discriminative properties, its evaluative properties are too inadequate for it to be used in cost/utility analyses. Poor correlation between the standard gamble and the rating scale indicates that utilities cannot be derived from rating scale data.
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