Comparison of disease-specific and a generic quality of life measure in patients with bronchial asthma.

BACKGROUND. Quality of life is being increasingly recognized as an important outcome in chronic and terminal illnesses. There are few publications from India on the characteristics of the instrument that measures quality of life in clinical trials. We describe a method for choosing an appropriate instrument in a randomized trial. METHODS. We selected thirty-two patients with bronchial asthma randomly and evaluated them to compare the validity and responsiveness of the disease-specific quality of life instrument in asthma (AQL) and the generic quality of life instrument, 'Sickness impact profile' (SIP), to detect changes in their health status. Validity was determined by a priori constructs (construct validation) and the responsive coefficient was calculated by determining the relationship to the 'minimal significant change in asthma score' and the 'variability' seen in this change in stable patients. RESULTS. The constructs used in validating the scores were that the change in quality of life score would correlate (i) highly with change in self-assessment of the disease (r > 0.7), (ii) moderately with change in physician assessment of the disease (r > 0.5), and (iii) minimally with change in peak flow reading (r > 0.3). We found both instruments to have good construct validity. The responsiveness coefficients noted for AQL and SIP were 1.8 (CI 0.65-3) and 0.7 (CI 0.3-1.2), respectively. CONCLUSIONS. Though both AQL and SIP were valid measures of quality of life, AQL is likely to be more capable of detecting smaller changes in the health status of patients with bronchial asthma and hence was chosen as the instrument in the proposed clinical trial.