18F-FDG PET-CT in rheumatoid arthritis patients tapering TNFi: reliability, validity and predictive value

Objectives To investigate the reliability and validity of fluorine-18 fluorodeoxyglucose (F-18-FDG) PET-CT scanning (FDG-PET) in RA patients with low disease activity tapering TNF inhibitors (TNFis) and its predictive value for successful tapering or discontinuation. Methods Patients in the tapering arm of the Dose REduction Strategies of Subcutaneous TNFi study, a randomized controlled trial of TNFi tapering in RA, underwent FDG-PET before tapering (baseline) and after maximal tapering. A total of 48 joints per scan were scored both visually [FDG-avid joint (FAJ), yes/no] and quantitatively [maximal and mean standardized uptake values (SUVmax and SUVmean)]. Interobserver agreement was calculated in 10 patients at baseline. Quantitative and visual FDG-PET scores were investigated for (multilevel) association with clinical parameters both on a joint and patient level and for the predictive value at baseline and the change between baseline and maximal tapering (Delta) for successful tapering and discontinuation at 18 months. Results A total of 79 patients underwent FDG-PET. For performance of identification of FAJs on PET, Cohen's kappa was 0.49 (range 0.35-0.63). For SUVmax and SUVmean, intraclass correlation coefficients were 0.80 (range 0.77-0.83) and 0.96 (0.9-1.0), respectively. On a joint level, swelling was significantly associated with SUVmax and SUVmean [B coefficients 1.0 (95% CI 0.73, 1.35) and 0.2 (0.08, 0.32), respectively]. On a patient level, only correlation with acute phase reactants was found. FDG-PET scores were not predictive of successful tapering or discontinuation. Conclusions Quantitative FDG-PET arthritis scoring in RA patients with low disease activity is reliable and has some construct validity. However, no predictive values were found for FDG-PET parameters for successful tapering and/or discontinuation of TNFi.