Agreement between measures of "walking-related knee pain" in knee osteoarthritis: a cross-sectional study

Abstract Introduction Modest effect sizes of clinical pain intervention studies have caused a need for evaluation of the applied methodology, including identification of treatment response indicators. Comparing two measurement instruments helps to identify the underlying constructs, which is important in interpreting the results appropriately, in research as well as in practice. This study aimed at assessing the agreement between a performance measure (walking task including pain assessment) and a patient reported outcome (the Knee injury and Osteoarthritis Outcome Score), in a population of patients with knee osteoarthritis. Materials and methods Cross-sectional data from 143 patients with knee osteoarthritis included in a prospective weight loss study were analysed. All participants completed the Knee injury and Osteoarthritis Outcome Score within one week prior to rating their target knee pain on a 100 mm visual analogue scale after walking 150–200 m at a self-selected pace in a gait laboratory. The Knee injury and Osteoarthritis Outcome Score pain subscale and item 5 of the Knee injury and Osteoarthritis Outcome Score pain subscale (“amount of knee pain experienced during walking on a flat surface in the the last week”) were selected for analysis. Distributions of visual analogue scale scores within the Knee injury and Osteoarthritis Outcome Score pain subscale 05 response categories were described and compared using Spearman correlation. Agreement was estimated using Limits of Agreement. Results There was a moderate correlation between visual analogue scale and Knee injury and Osteoarthritis Outcome Score pain subscale 05 (r = 0.5, p < 0.001), with a wide range of visual analogue scale scores within the Knee injury and Osteoarthritis Outcome Score response categories. Generally, higher pain scores were reported with the Knee injury and Osteoarthritis Outcome Score pain subscale than with the performance test, with a mean difference of 18.8 (SD 16.6), and Limits of Agreement from –13.6 to 51.3. Conclusion Disagreement between the performance measure and the Knee injury and Osteoarthritis Outcome Score pain subscale, together with the moderate correlation of visual analogue scale and Knee injury and Osteoarthritis Outcome Score pain subscale 05 scores, suggests a difference in the underlying constructs of pain.