AIMS: Recent research has shown that it may not be valid to compare scores across diseases using generic outcome scales. An alternative approach is now available: Scores from two different disease-specific measures can be co-calibrated onto the same scale using Rasch analysis as long as the measures are based on the same measurement model. The aim of the study was to investigate whether it is feasible to co-calibrate scores for patients with different skin diseases using measures specific to psoriasis and atopic dermatitis.;
METHODS: The Psoriasis Quality of Life (PsoriQoL) scale and the Quality of Life in Atopic Dermatitis (QoLIAD) scale were both based on the needsbased model of quality of life (QoL). Each was developed using Rasch analysis and provides unidimensional measurement. The scales share 5 common items allowing co-calibration of scores using a common item design. Initially, the scales were analysed separately for fit to the Rasch model. Problematic items were removed. The scales were then combined and assessed again for fit to the model. Common items were assessed for Differential Item Functioning (DIF) by disease.;
RESULTS: The sample consisted of 146 psoriasis (male 50 %, mean age = 44.2 range = 17–83 years) and 146 atopic dermatitis patients (male 50 %, mean age = 45.5, range = 20–82 years). Both scales showed adequate overall fit to the Rasch model (PsoriQoL: Chi2 = 56.45, df = 50, p = 0.25; QoLIAD: Chi2 = 55.34, df = 50, p = 0.28). Evidence of multidimensionality was present in the QoLIAD. One item was removed due to high residual correlations with 2 others, improving model fit statistics. One item in each scale showed misfit and these were removed. After combing the items one common item showed DIF by disease and was also removed. The final set of items showed excellent fit to the model (Chi2 = 99.20, df = 84, p = 0.12).;
CONCLUSIONS: It is now possible to provide valid comparative QoL scores for patients with different diseases. Such comparisons cannot be made meaningfully using generic PROs that also tend to have poor measurement properties.
