Rasch analysis of a new Patient reported outcome measure for Psoriasis Treatment (PROMPT)

OBJECTIVESA draft patient-reported outcome measure for psoriasis treatment (PROMPT) was developed through patient interviews and comprised 91 items across seven core domains. This study aimed to evaluate the scaling properties and construct validity of the draft measure using the Rasch measurement model.METHODSPatients with chronic plaque psoriasis were identified and recruited according to pre-defined inclusion and exclusion criteria through psoriasis-specific secondary care clinics in the United Kingdom and two national patient organizations. Patients completed the draft measure at two time points, 14 days apart. Respondents with ≥40% of missing data were removed from the final analyses. Data from each sub-scale were analyzed separately using RUMM2030 software to explore Rasch model fit, item difficulty, local dependence, item category thresholds, and differential item functioning (DIF) by age and gender. Where necessary, items were removed individually and the scale iteratively reassessed for fit and unidimensionality.RESULTSA sample of n=209 patients with chronic plaque psoriasis completed the draft measure. Initial fit to the model was poor; disordered category thresholds were identified for items in all scales. Post-hocre-scoring from a 5-point Likert scale to a 3-point Likert scale improved model fit. Items which showed local dependence were removed in context of qualitative findings. Following removal of 11 items, all 7 scales demonstrated acceptable fit with the Rasch model (Chi Sq = 0.09 to 0.2) There was no evidence of DIF by age and gender.CONCLUSIONSThe new measure, PROMPT, comprised 80 items in 6 independent, unidimensional scales, free from age or gender bias, with acceptable fit to the Rasch model. As such, the measure is considered to show initial promise for use with patients with chronic plaque psoriasis in a clinical setting. The psychometric properties and scoring of the measure should be explored further and confirmed in future studies.