T.P.39 : Psychometric properties of ACTIVLIM in 2-year cross sectional records of the Belgian Neuromuscular Disease Registry

Abstract

ACTIVLIM is a Rasch-built measure of activity limitations in children and adults with neuromuscular disorders. This study aims to investigate its psychometric properties as used in clinical practice in the Belgian NeuroMuscular Disease Registry (BNMDR). A sample of 2986 consecutive patients (56 % male, 92 % adult over 16 years, and 85 % Dutch speakers) was assessed at least once in the BNMDR over years 2011 and 2012 in 6 reference centers across the three main regions in Belgium, leading to 4146 records. The dataset was analyzed with the Rasch rating scale model in order to determine the difficulty of each item (i.e. the calibration) and their targeting to the ability of the patients on a unidimensional and linear scale, to investigate item and person fit and item invariance across demographic and clinical sub-groups, to determine the reliability in this dataset, to compare the BNMDR item difficulty hierarchy with the original scale calibration and to measure the patients change in activity (t-score) over on year (n = 1128). The BNMDR ACTIVLIM data showed excellent fit to a unidimensional scale. The reliability index was 0.95 in the BNMDR and the items were well targeted for 87 % of the patients. Invariant item difficulties were observed across age, gender, language, regions, centers, chronicity, item presentation orders and over time. Slight clinically meaningful variations in item difficulty hierarchy were observed across diagnoses. The item calibration was similar to the original publication but with a 3-fold accuracy. A slight but significant deterioration in activity (t = 0.39 ± 1.70; p 0.1). The ACTIVLIM records in the BNMDR demonstrated very good psychometric properties.