The Interrater Reliability of the Greek Expanded and Revised Gross Motor Function Classification System and the Family Report Questionnaire in Cerebral Palsy

Background/Objectives: The Gross Motor Function Classification System–Expanded & Revised (GMFCS-E&R) is widely used to describe gross motor performance in children with cerebral palsy (CP). Although Greek-language materials are available, interrater reliability across healthcare professionals and parents has not been examined. This study evaluated the reliability of the Greek GMFCS-E&R among a pediatric neurologist, a pediatric physiotherapist, and parents, with an emphasis on descriptors and illustrations for older children and adolescents. Methods: A cross-sectional study was conducted with 111 children and adolescents with CP aged 2–18 years. Professionals classified each child using the Greek GMFCS-E&R brochure (ages 2–6) or the descriptors and illustrations (ages 6–12 and 12–18). Parents completed the age-appropriate GMFCS Family Report Questionnaire. Agreement among the three raters was assessed using Fleiss’ kappa (κF), and pairwise agreement using weighted Cohen’s kappa (κCw), overall and by age band. Results: Overall interrater reliability was substantial (κF = 0.77). Agreement by GMFCS level ranged from κF = 0.68 (Level V) to κF = 0.85 (Level I). Reliability increased with age, reaching κF = 0.74–0.85 in adolescents. Pairwise agreement was excellent across all rater pairs, with near-perfect concordance between the pediatric neurologist and physiotherapist (κCw = 0.98). In >60% of disagreements, parents assigned higher levels, typically between adjacent categories. Conclusions: The Greek-language GMFCS-E&R demonstrates high interrater reliability among healthcare professionals and parents, with excellent agreement when using descriptors and illustrations for older children and adolescents. The GMFCS-FR effectively incorporates parental perspectives and complements clinical assessment, supporting the use of the Greek GMFCS-E&R in routine clinical practice and research settings.