Measurement properties of the Inflammatory Rasch-built Overall Disability Scale (I-RODS) in patients with Guillain–Barré syndrome

Guillain–Barré syndrome (GBS) is an acute immune-mediated polyneuropathy with a high disease impact, even after good clinical recovery. The Inflammatory Rasch-built Overall Disability Scale (I-RODS) is a Patient-Reported Outcome Measure (PROM) developed for patients with immune-mediated neuropathies that measures limitations in daily activities and social participation. It consists of 24 items, scored from 0–48. The present study aimed to validate the measurement properties of the I-RODS in patients with GBS included in the prospective International GBS Outcome Study (IGOS). The current study focussed on structural validity, cross-cultural validity, internal consistency, and construct validity of I-RODS using Rasch-based methods. The study was conducted in 1226 patients diagnosed with GBS with a median I-RODS score of 28 (IQR 10–41) 4 weeks after inclusion into IGOS. Rasch analyses revealed adequate internal consistency (PSI = 0.95; α = 0.98) and sufficient construct validity, indicated by strong correlations (R = − 0.91 to − 0.77). Targeting was acceptable, although there was a skew towards the floor (10.1%) and ceiling (9.4%). However, the I-RODS showed poor unidimensionality (12.1% CI 10.7–13.5%) and poor overall fit to the Rasch model. Category thresholds were correctly ordered. Misfit was found in two items. Additionally, 15 of 276 item pairs showed local dependency. While no differential item functioning (DIF) was evident for age or sex, significant DIF by geographic region was observed, with the strongest DIF in one item. These results suggest that the current version of I-RODS could be improved or alternative PROMs for patients with GBS could be developed.