Criterion validity of the GMFM‐66 item set and the GMFM‐66 basal and ceiling approaches for estimating GMFM‐66 scores

AIM: The aim of this study was to compare the accuracy of two abbreviated approaches for estimating Gross Motor Function Measure 66 (GMFM-66) scores against the full GMFM-66 and to explore their strengths and limitations. METHOD: An existing dataset (n=224) comprising children aged 1 to 13 years (mean age 6y 11mo, SD 4y 6mo; 132 males, 92 females) with cerebral palsy (CP) of all Gross Motor Function Classification System (GMFCS) levels was used to compare the validity of the item set version (GMFM-66-IS) and the basal and ceiling version (GMFM-66-B&C) with the full GMFM-66 scores. Follow-up assessment at 1 year (n=109) allowed evaluation of change scores and accuracy at a single point in time. RESULTS: The cross-sectional agreement was excellent for both abbreviated measures (all intraclass correlation coefficients [ICCs] >0.98). When measuring change over time, both the GMFM-66-IS and the GMFM-66-B&C showed good agreement for children with bilateral CP (ICCs >0.9). However, the GMFM-66-IS assessed change over 1 year more accurately than the GMFM-66-B&C in children with unilateral CP (ICC=0.89 vs ICC=0.58; 95% confidence intervals do not overlap). INTERPRETATION: Both approaches for estimating GMFM-66 scores are accurate at a single point in time. If the primary goal of assessment is to measure change, the full GMFM-66 should still be regarded as the criterion standard. The GMFM-66-IS should be the preferred shortened measure for children with unilateral CP.