Minimally important change and smallest detectable change of the OSTRC questionnaire in half- and full-marathon runners.

The purpose of this study was to evaluate the smallest detectable change (SDC), minimally important change (MIC), and factor structure of the Oslo Sports Trauma Research Center (OSTRC) questionnaire severity score in half- and full-marathon runners. Data came from a prospective cohort study, the SUcces Measurement and Monitoring Utrecht Marathon (SUMMUM) 2017 study. Two external anchors, the global rating of change (GRC) and global rating of limitations (GRL), were used to classify the running-related injuries (RRI) as truly improved, unchanged, or truly worsened. SDC values were calculated at individual and group levels. MIC values were calculated using the visual anchor-based MIC distribution and mean change methods. Confirmatory factor analysis (CFA) was used to study the a priori hypothesized factor structure. A total of 132 runners who reported the same RRI on two occasions 2 weeks apart were included in the analysis. SDC values at individual and group levels were ≤35.06 and ≤9.30, respectively. With the visual anchor-based MIC distribution method, the MIC values for RRIs that truly improved according to the GRC and GRL anchors were 13.50 and 18.50, respectively. With the mean change method, the MIC values for RRIs that truly improved according to the GRC and GRL anchors were 15.49 and 45.38, respectively. The CFA confirmed that the OSTRC was a unidimensional questionnaire. The change score of the OSTRC severity score can be used to distinguish between important change and measurement error at a group level using the MIC value 18.50. Because the SDC of the OSTRC severity score was larger than the MIC, it is not advised to use the MIC at an individual level.