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

OBJECTIVES To investigate the reliability and validity of 18F-FDG PET-CT scanning (FDG-PET) in rheumatoid arthritis (RA) patients with low disease activity tapering tumor necrosis factor inhibitors (TNFi) and its' predictive value for successful tapering or discontinuation. METHODS Patients in the tapering arm of the DRESS study, a randomized controlled trial on TNFi tapering in RA, underwent FDG-PET before tapering (baseline) and after maximal tapering. 48 joints per scan were scored: 1) visually (FDG-avid joint (FAJ) y/n), 2) 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: 1) (multilevel) association with clinical parameters both on joint and patient level and 2) predictive value at baseline and change between baseline and maximal tapering (delta) for successful tapering and discontinuation at 18 months. RESULTS 79 patients underwent FDG-PET. For performance of identification of FAJs on PET, Cohen's kappa was 0.49 (0.35-0.63). For SUVmax and SUVmean, ICCs were 0.80 (0.77-0.83) and 0.96 (0.9-1.0), respectively. On joint level, swelling was significantly associated with SUVmax and SUVmean (B coefficients with 95%CI 1.0 (0.73-1.35) and 0.2 (0.08-0.32) respectively). On patient level only correlation with acute phase reactants was found. FDG-PET scores were not predictive for 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.