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Supplemental Digital Content is available in the text. Background: 18F-Fluorodeoxyglucose (FDG) positron-emission tomography/computed tomography (PET/CT) was recently introduced as a new tool for the diagnosis of prosthetic heart valve endocarditis (PVE). Previous studies reporting a modest diagnostic accuracy may have been hampered by unstandardized image acquisition and assessment, and several confounders, as well. The aim of this study was to improve the diagnostic performance of FDG PET/CT in patients in whom PVE was suspected by identifying and excluding possible confounders, using both visual and standardized quantitative assessments. Methods: In this multicenter study, 160 patients with a prosthetic heart valve (median age, 62 years [43–73]; 68% male; 82 mechanical valves; 62 biological; 9 transcatheter aortic valve replacements; 7 other) who underwent FDG PET/CT for suspicion of PVE, and 77 patients with a PV (median age, 73 years [65–77]; 71% male; 26 mechanical valves; 45 biological; 6 transcatheter aortic valve replacements) who underwent FDG PET/CT for other indications (negative control group), were retrospectively included. Their scans were reassessed by 2 independent observers blinded to all clinical data, both visually and quantitatively on available European Association of Nuclear Medicine Research Ltd–standardized reconstructions. Confounders were identified by use of a logistic regression model and subsequently excluded. Results: Visual assessment of FDG PET/CT had a sensitivity/specificity/positive predictive value/negative predictive value for PVE of 74%/91%/89%/78%, respectively. Low inflammatory activity (C-reactive protein <40 mg/L) at the time of imaging and use of surgical adhesives during prosthetic heart valve implantation were significant confounders, whereas recent valve implantation was not. After the exclusion of patients with significant confounders, diagnostic performance values of the visual assessment increased to 91%/95%/95%/91%. As a semiquantitative measure of FDG uptake, a European Association of Nuclear Medicine Research Ltd–standardized uptake value ratio of ≥2.0 was a 100% sensitive and 91% specific predictor of PVE. Conclusions: Both visual and quantitative assessments of FDG PET/CT have a high diagnostic accuracy in patients in whom PVE is suspected. FDG PET/CT should be implemented early in the diagnostic workup to prevent the negative confounding effects of low inflammatory activity (eg, attributable to prolonged antibiotic therapy). Recent valve implantation was not a significant predictor of false-positive interpretations, but surgical adhesives used during implantation were. [ABSTRACT FROM AUTHOR]