Rasch-Built Overall Disability Scale for Chronic Idiopathic Axonal Polyneuropathy

BACKGROUND AND OBJECTIVES: Chronic idiopathic axonal polyneuropathy (CIAP) is a common type of chronic polyneuropathy that often affects health-related quality of life. Currently, no patient-reported disease-specific outcome measure is available to assess functional deficit in patients with CIAP. The aim of this study was to construct a patient-reported Rasch-built interval scale for patients with CIAP. METHODS: A historic prospective cohort study was conducted to develop a CIAP-specific Rasch-built overall disability scale (CIAP-RODS). The preliminary scale (pre-CIAP-RODS) comprised 196 items, including 146 activity and participation items selected from the World Health Organization International Classification of Functioning, Disability and Health and 50 expert-derived items formulated in collaboration with a CIAP patients advocacy group. Participants were patients with CIAP who were requested to score their perceived difficulty to perform each item as (0) unable to perform; (1) able to perform, but with difficulty; or (2) easily performed, without difficulty. For test-retest reliability studies, 150 participants completed the pre-CIAP-RODS twice with an interval of 2-4 weeks. The pre-CIAP-RODS was subjected to Rasch analyses (RUMM2030+) to develop the final CIAP-RODS, and external validity was assessed by examining associations with the European Quality of Life 5 Dimensions 3 Level Version (EQ-5D-3L). RESULTS: Of 551 eligible patients invited, 268 completed the pre-CIAP-RODS (mean age 72 years; 31% female; median disease duration 14 years). Patients with relevant comorbidities or newly identified risk factors of polyneuropathy were excluded. The pre-CIAP-RODS was subjected to Rasch analyses and did not meet the Rasch model expectations. In a systematic and stepwise manner, items were removed based on disordered thresholds, misfit statistics, local dependency, previously reported patient perceptions, and clinical applicability, resulting in a final 24-item CIAP-RODS that fulfilled Rasch model requirements. Test-retest reliability was good, internal validity was robust (person separation index 0.95), and significant associations between CIAP-RODS person location and EQ-5D-3L item scores indicated good discriminative external validity. DISCUSSION: The 24-item CIAP-RODS is a disease-specific interval measure developed for detection of activity and participation limitations in patients with CIAP. Use of the CIAP-RODS in future studies is recommended to evaluate longitudinal changes in the disease course. Further studies are needed to determine responsiveness and cross-cultural validity.