Observational skills assessment score: reliability in measuring amount and quality of use of the affected hand in unilateral cerebral palsy

Background: The Observational Skills Assessment Score (OSAS) measures amount and quality of use of the affected hand in children with unilateral Cerebral Palsy (CP) in bimanual activities and could therefore be a valuable addition to existing assessment tools. The OSAS consists of tasks that are age appropriate and require use of the affected hand. Methods: To measure the agreement and reliability of the OSAS a convenience sample of two groups of 16 children with unilateral spastic CP (2.5-6 and 12-16 years old), performed age specific bimanual tasks in 2 measurement sessions. Three experienced raters took part in testing and 8 in scoring. Intra class correlation (ICC) values for intra-and inter-rater reliability, and the mean and standard deviation of the differences between measurements were calculated. For test-retest reliability beside ICC scores, Smallest Detectable Differences (SDDs) were calculated in 16 older and 10 younger children. Results: Generally, there seems to be good agreement between repeated measurements of the OSAS, as indicated by the small SDDs on most scales for quality of movement, compared to the range of their scales. This indicates potentially good sensitivity to change if used for patient evaluation purposes. The exceptions were the 'quality of reach' score for all tasks, and all quality scores for the stacking blocks task for the young children. As used in the present study, the OSAS has good discriminative capacity within patient populations as indicated by the high ICCs for most quality scores. Measuring the amount of use does not seem to be useful for either discrimination or evaluation. Conclusion: In general, the OSAS seems to be a reliable tool for assessing the quality of use of the affected hand in bimanual activities in younger and older children with unilateral CP. Some modifications may improve its usefulness and efficiency.