Harmonisation of the 0-10 Numerical Rating Scale for pain intensity and the pain domain of the Western Ontario and McMaster Universities Osteoarthritis Index

BackgroundAnalgesic efficacy is often evaluated with patient-reported outcome measures (PROMs). However, many different PROMs are used, which poses a problem when results are pooled in meta-analyses. Two commonly used PROMs used for evaluating pain intensity are the 0-10 numerical rating scale (NRS) and the pain domain of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). ObjectiveIn this study, we aim to evaluate if the NRS and the WOMAC pain domain can be harmonised and, if possible, to report a conversion table. MethodsThe study is based on a large dataset of 12-to 18-month pain outcomes after primary total hip arthroplasty (THA), total knee arthroplasty (TKA), or unicompartmental knee arthroplasty (UKA) for osteoarthritis (ClinicalTrials.gov identifiers NCT05845177 and NCT05900791). We will apply multiple imputation to impute missing WOMAC pain domain responses. We will assess the distribution of scores and evaluate if NRS and the WOMAC pain domain have sufficient correlation and a monotonous relationship. We will also assess the NRS and WOMAC pain domain relationship within important subgroups, such as age, sex, and type of surgery. If deemed appropriate, we will perform equipercentile linking to create a conversion table from WOMAC pain domain sum scores to NRS scores and vice versa. We will assess the relationship between predicted and actual values with root mean squared errors (RMSEs) and mean absolute errors (MAEs), concordance correlation coefficients (CCCs) and intraclass correlation coefficients (ICC), Bland-Altman plots, residual plots, and calibration plots. PerspectiveThe results will be submitted for publication in a peer-reviewed journal. We will seek to make the reports freely available, either by open-access publication or through publication on a preprint server, e.g. www.medrxiv.org.