Associations between single-question Visual Analogue Scale pain score and weight-bearing and non-weight-bearing domains of Western Ontario and McMaster Universities Arthritis Index pain: data from 2 phase 3 clinical trials

Introduction: Visual Analogue Scale (VAS) and the pain subscale of the Western Ontario and McMaster Universities Arthritis Index (WOMAC) are commonly used measuring tools of osteoarthritis (OA) pain. Objectives: The objective of this cross-sectional study was to explore the associations between single-question VAS pain and the weight-bearing and non-weight-bearing domains of WOMAC pain. Methods: Data from 2093 patients with OA participating in 2 phase 3 clinical trials were included for post hoc analyses. Univariate Pearson correlations and comparison of r values were made using z statistics obtained using the Fisher r to z test for all items of the VAS pain scale, the WOMAC pain subscale, the weight-bearing and non-weight-bearing constructs of WOMAC pain subscale, and by subgroups of WOMAC pain quintiles and Kellgren-Lawrence grades. Results: The correlations between VAS pain and WOMAC pain were significant (r = 0.67, P < 0.001) with a slope of 0.57 (95% confidence interval [CI]: 0.54-0.61). A similar correlation was found for weight-bearing pain (r = 0.68, P < 0.001, slope: 0.62 (95% CI: 0.59-0.65) but significantly lower for non-weight-bearing pain (r = 0.55, P < 0.001, slope: 0.49 (95% CI: 0.46-0.52). The degree of disagreement between the 2 instruments seemed to be lesser in the extreme ends of the scales, and the observed association between Kellgren-Lawrence grade and disagreement between VAS and WOMAC was driven by non-weight-bearing pain. Conclusion: In conclusion, VAS pain and WOMAC pain subscale correlation was found to be moderate and the VAS pain scale correlated more accurately with the WOMAC pain weight-bearing questions. This constitutes novel insight into patient with OA pain reporting.