The objective of this study was to examine the influence of different analytical methods, baseline covariates, followup periods, and anchor questions when establishing a minimal important difference (MID) for individuals with knee osteoarthritis (OA). Second, to propose MID for improving and worsening on the Knee injury and Osteoarthritis Outcome Score (KOOS). Retrospective analysis of prospectively collected data from 272 patients with knee OA undergoing a multidisciplinary nonsurgical management strategy. The magnitude and rate of change as well as the influence of baseline covariates were examined for 5 KOOS subscales over 52 weeks. The MID for improving and worsening were investigated using 4 anchor-based methods. Waitlisted for joint replacement and exhibiting unilateral/bilateral symptoms influenced change in KOOS over time. Generally, low correlations between anchors and KOOS change scores limited calculations of MID; thus, they were only proposed for the pain, activities of daily living, and quality of life subscales. The method used to calculate the MID influenced the cutpoint; however, the type of anchor question only influenced the MID when analyzed with a particular mean change method. Depending on patient and clinical characteristics, the subscale, and the analytical approach used, the MID for KOOS improvement ranged from an absolute change of -1.5 to 20.6 points and worsening ranged from -19.17 to 8.5 points.
MID vary with patient and clinical characteristics, KOOS subscale, and analytical approach. Provided the anchor question is relevant to the patient-reported outcome and baseline status is considered, the anchor does not appear to influence the MID for improvement or worsening when using some anchor-based methods.