Living with Chronic Illness Scale: international validation through the classic test theory and Rasch analysis among Spanish‐speaking populations with long‐term conditions

Background: The Living with Chronic Illness (LW-CI) Scale is a comprehensive patient-reported outcome measure that evaluates the complex process of living with long-term conditions. Objective: This study aimed to analyse the psychometric properties of the LW-CI scale according to the classic test theory and the Rasch model among individuals living with different long-term conditions. Design: This was an observational, international and cross-sectional study. Methods: A total of 2753 people from six Spanish-speaking countries living with type 2 diabetes mellitus, chronic obstructive pulmonary disease, chronic heart failure, Parkinson's disease, hypertension and osteoarthritis were included. The acceptability, internal consistency and validity of the LW-CI scale were analysed using the classical test theory, and fit to the model, unidimensionality, person separation index, item local independency and differential item functioning were analysed using the Rasch model. Results: Cronbach's α for the LW-CI scale was.91, and correlation values for all domains of the LW-CI scale ranged from.62 to.68, except for Domain 1, which showed correlation coefficients less than.30. The LW-CI domains showed a good fit to the Rasch model, with unidimensionality, item local independency and moderate reliability providing scores in a true interval scale. Except for two items, the LW-CI scale was free from bias by long-term condition type. Discussion: After some adjustments, the LW-CI scale is a reliable and valid measure showing a good fit to the Rasch model and is ready for use in research and clinical practice. Future implementation studies are suggested. Patient and Public Contribution: Patient and public involvement was conducted before this validation study - in the pilot study phase.