Agreement between a generic and disease-specific quality-of-life instrument: the 15D and the SGRQ in asthmatic patients.

The generic health-related quality of life (HRQOL) instrument 15D, and the disease-specific St. George's Respiratory Questionnaire (SGRQ) were highly correlated in cross-sectional and in follow-up data among asthmatic patients. However, a high correlation does not mean that these instruments yield similar results about outcome. We measured among 134 asthmatic patients the mathematical relationship between the changes in the scores of these instruments by regression techniques and agreement by cross-tabulation. The prospective data included measurements at baseline and at one, three and five years. The three measurements of change in HRQOL were compared with the baseline. There was a close mathematical relationship between the scores, but it was affected significantly by confounding factors such as patient's age, gender and smoking habits. The instruments agreed on the direction of change in HRQOL (positive or negative) in 64.8% of patients, while in 15.8% the changes were in opposite direction. There was a statistically significant difference in indicating the direction of changes (improvement or decline) of these instruments. The changes in the SGRQ scores agreed slightly better than those in the 15D with the changes in clinical parameters. The conclusion was that these instruments do not necessarily yield similar results of effectiveness. Therefore, the choice of HRQOL instrument may influence the economic attractiveness of different asthma interventions. The disease-specific SGRQ agreed slightly better than the generic 15D with the changes in clinical parameters.