Observer variability in the assessment of renal (18)F-FDG uptake in kidney transplant recipients

(18)F-FDG PET/CT imaging may help non-invasively disprove the diagnosis of acute kidney allograft rejection (AR) in kidney transplant recipients (KTR). The present study aims at evaluating the repeatability and reproducibility of the quantification of renal (18)F-FDG uptake in KTR. We prospectively performed (18)F-FDG PET/CT in 95 adult KTR who underwent surveillance transplant biopsy between 3 to 6 months post transplantation. Images were obtained 180 minutes after injecting 3 MBq (18)F-FDG per kg body weight. Mean standard uptake value (SUV(mean)) of kidney cortex was independently measured by 2 experienced observers in 4 volumes of interest (VOI) distributed in the upper (n = 2) and lower (n = 2) poles. The first observer repeated SUV assessment in the uppermost VOI, blinded to the initial results. Intra-class correlation coefficients (ICC) and Bland-Altman plots were calculated. An ICC of 0.96 with 95%CI of [0.94; 0.97] was calculated for the intra-observer measurements. The ICC for inter-observer reproducibility for each VOI was 0.87 [0.81–0.91], 0.87 [0.81–0.91], 0.85 [0.78–0.89] and 0.83 [0.76–0.88] for the upper to the lower renal poles, respectively. The repeatability and reproducibility of the quantification of kidney allograft (18)F-FDG uptake are both consistent, which makes it transferrable to the clinical routine.