Longitudinal reference centiles for the Gross Motor Function Measure‐66 in children and adolescents with cerebral palsy

Aim To establish novel longitudinal reference values for the Gross Motor Function Measure‐66 (GMFM‐66) in children and adolescents with cerebral palsy aged 3 to 18 years, to enable more accurate assessments of changes in motor function. Method This was a single‐centre retrospective analysis of patients who participated in a rehabilitation programme between January 2006 and March 2022. The GMFM‐66 was used to measure gross motor function. Paired GMFM‐66 measurements from the follow‐up phase of the rehabilitation programme were used to establish a reference centile for the change in GMFM‐66 over a 6‐month period using the lambda–mu–sigma method. Results Reference centiles for GMFM ‐66 changes (over a 6‐month period; ±1 month) were created using 1190 longitudinal data pairs of GMFM ‐66 measurements (mean age 8 years 4 months [standard deviation 7 years 11 months] at start of follow‐up), Gross Motor Function Classification System levels I to V. The z‐scores for GMFM ‐66 change of a validation dataset by the new tool and the previously described method to quantify a change in GMFM ‐66 by individual effect size were highly correlated (Pearson's rank correlation coefficient 0.981 [95% confidence interval 0.979–0.984], p < 0.001) Interpretation The new reference values showed a high correlation with the previously published reference values, which were limited to an age range of 3 to 12 years. The new reference values can be applied from an age of 3 to 18 years. This facilitates the evaluation of medical treatment after a 6‐month period also for children with cerebral palsy who are older than 12 years.