OBJECTIVES: Disease-specific patient-reported outcomes (PROs) are designed to be highly relevant to a single disease. It is widely believed that comparisons of outcomes between patients with different diseases is only possible using generic PRO measures. The present study employs a novel method of using Rasch analysis to co-calibrate scores from different disease-specific PROs allowing scores to be compared across diseases.
METHODS: Three samples of patients completed the Asthma Life Impact Scale (ALIS), the Living with COPD scale (LCOPD) or the Cambridge Pulmonary Hypertension Outcome Review (CAMPHOR), depending on their illness. Each scale utilises the needs-based model of QoL and the scales share 8 common items. The three samples were analysed separately for fit to the Rasch model and then combined and re-analysed.
RESULTS: The ALIS was completed by 140 asthma patients (mean age=50.6, males = 29.3%); the LCOPD by 162 COPD patients (mean age=69.3, males = 43.8%) and the CAMPHOR by 91 patients (mean age=52.6, males = 29.7%). Each of the scales fit the Rasch model individually (ALIS Chi2 = 0.05; LCOPD Chi2 = 0.32; CAMPHOR Chi2 = 0.92). The combined dataset also fit the Rasch model at first run (Chi2 = 0.24). One common item showed misfit (Chi2 less than 0.001) and non-uniform differential item functioning (DIF) by disease (Chi2 less than 0.001). This item was removed from the analysis and the final co-calibrated scale showed good fit to the Rasch model (Chi2=0.48) with minimal DIF by age, gender or disease.
CONCLUSIONS: The results showed that it was possible to co-calibrate scores on the ALIS, LCOPD and CAMPHOR. As disease-specific measurement has advantages over generic assessment related to relevance and reproducibility, the results have the potential to enhance PRO measurement in respiratory research.
