Psychometric Properties of the Asthma Symptom Diary (ASD), a Diary for Use in Clinical Trials of Persistent Asthma

Background No currently available asthma symptom diary has sufficient validation to be recommended for use as a core asthma outcome measure. Objective The objective of this study was to provide validation data for the 10-item asthma symptom diary (ASD). Methods Data were collected in a 4-week prospective, observational study. Subjects completed 3 study visits, completing the ASD twice daily at home for 28 days. Psychometric properties in terms of dimensionality, reliability, validity, and responsiveness were assessed. Results Data from 276 subjects were analyzed; mean age was 42.9 (standard deviation [SD] = 16.4) years, mean asthma duration was 23.3 (SD = 16.8) years, and 69.6% were female. Confirmatory factor and Rasch analysis supported the ASD as unidimensional and adequately measuring the spectrum of asthma symptom severity. High Cronbach's α (0.94) and intraclass correlation coefficients (0.89-0.95) supported reliability. A high correlation between the 7-day average ASD score and the Asthma Control Questionnaire (ACQ) total score ( r = 0.75) and Asthma Quality of Life Questionnaire total scores ( r = −0.76), and a moderate correlation with FEV1 % predicted ( r = −0.30) supported convergent validity. Significant differences ( P < .001) between groups classified by ACQ scores supported known-group validity. The 7-day average ASD scores were responsive to change, with significantly higher score changes ( P < .001) in responders versus nonresponders. Minimally important differences were calculated and found to be in the range of 0.1-0.3. Conclusion Results of this study indicated that the ASD is a reliable and valid asthma symptom measure for use in adult and adolescent asthma patients to evaluate the effect of treatment on asthma in clinical trials.