Antibiotic-Associated Overanticoagulation Unmasking Prosthetic Valve Endocarditis Diagnosed by Fluorodeoxyglucose-Positron Emission Tomography/Computed Tomography (FDG-PET/CT)

Vitamin K antagonists (VKAs) remain the standard anticoagulation therapy in patients undergoing mechanical valve replacement; despite their efficiency in this clinical context, they are associated with a very narrow therapeutic window, a factor that links them to a major risk of bleeding. Their broad involvement in the coagulation cascade and their interactions with multiple commonly used drugs can lead to supratherapeutic anticoagulation. Furthermore, prosthetic valve infective endocarditis is a serious complication that poses a diagnostic challenge, especially when echocardiographic findings are inconclusive. We present the case of a 50-year-old man with a mechanical aortic valve prosthesis secondary to severe aortic stenosis, on anticoagulation therapy with acenocoumarol, who presented with severe over-anticoagulation, with multiple hemorrhagic complications, including hemothorax, chronic subdural hematoma, and extensive intramuscular hematomas involving the iliopsoas muscle and deep compartments of the thigh. The onset of these complications was temporally associated with the administration of an antibiotic regimen containing amoxicillin, secondary to an upper respiratory tract infection. During his hospital stay, persistent fever and the isolation of Enterococcus faecalis from central and peripheral blood cultures raised suspicion of probable infective endocarditis; however, both transthoracic and transesophageal echocardiograms were inconclusive for vegetations. Therefore, positron emission tomography (PET) with fluorodeoxyglucose (FDG) combined with computed tomography (CT) was performed, revealing focal hypermetabolic uptake around the prosthetic aortic valve annulus, consistent with prosthetic valve endocarditis; targeted antimicrobial therapy was initiated, and the patient was subsequently referred for surgical valve replacement, which was performed after the completion of antibiotic treatment with satisfactory clinical progress. This case highlights the vast clinical consequences of anticoagulation instability associated with VKAs, and especially the exponential increase in risk due to drug interactions, as in this case, linked to antibiotic therapy. Furthermore, it emphasizes the value of FDG-PET/CT in the diagnosis of prosthetic valve endocarditis, particularly when echocardiographic findings are inconclusive.