Patient-assessed measures of health outcome in asthma: A comparison of four approaches

The study compares the psychometric properties of four different approaches to patient-assessed health outcomes in asthma, including the Asthma Qualit y of Life Questionnaire (AQLQ), Newcastle Asthma Symptoms Questionnaire (NA SQ), SF-12 and EuroQol. The instruments were administered by means of a sel f-completed postal questionnaire to 394 patients recruited from general pra ctices in the North East of England. Patients completed a follow-up questio nnaire at 6 months. The levels of missing data were assessed and instrument scores compared using correlational analysis. Scores were related to self- reports of smoking behaviour, socioeconomic status and health transition. R esponsiveness was assessed using standardized response means. Two hundred a nd thirty-five patients took part in the study giving a response rate of 59 .6%. There was a relatively large amount of missing data for the individual ized section of the A.QLQ. Correlational analysis provided evidence of conv ergent validity between the specific instruments; the largest correlation w as found between NASQ scores and the asthma symptoms scale of the AQLQ (r = 0.84). The NASQ was found to be the most powerful at discriminating betwee n smokers and non-smokers. All four instruments were linearly related to se lf-reported asthma transition (P < 0.05); the specific instruments having t he strongest association. The specific instruments showed good levels of re sponsiveness with the NASQ producing a large SRM of 0.82. SRMs for the AQLQ were of a moderate to large size (0.32-0.77) and the SRMs for the SF-12 an d EuroQol were of a small size. The two specific instruments are capable of greater levels of discrimination between groups of patients and are more r esponsive to changes in health than the generic SF-12 and EuroQol. The grea ter responsiveness of the NASQ is probably due to its focus being restricte d to symptoms of asthma compared to the broader focus of the AQLQ domains. The NASQ has a strong relationship with the AQLQ and is a more practical in strument that is more acceptable to patients. However, the AQLQ does measur e broader patient concerns. The SF-12 and EuroQol have greater potential to capture side-effects and have wider scope for application in economic eval uation.