Unusual Suspect: Infective Endocarditis Masquerading as ANCA-Associated Vasculitis

Introduction: ANCA-associated vasculitis is a small-vessel vasculitis often associated with rapidly progressive glomerulonephritis. However, infectious endocarditis can present with similar clinical and serologic findings, including positive ANCA and crescentic GN, leading to diagnostic confusion. Distinguishing between these entities is critical, as misdiagnosis can lead to inappropriate immunosuppression and poor outcomes. Case Description: A 79-year-old man with prior TAVR presented with fatigue, weight loss, and worsening renal function. Labs showed positive p-ANCA and rising creatinine; he was empirically started on steroids for suspected ANCA-associated vasculitis. He progressed to dialysis. Blood cultures later grew Streptococcus gordonii, and TEE revealed bioprosthetic valve vegetations, confirming endocarditis. Steroids were stopped, and he completed antibiotics. His course was complicated by septic embolic stroke and discitis. Discussion: This case underscores the diagnostic challenge in differentiating ANCA-associated vasculitis from infection-related glomerulonephritis due to endocarditis. Though ANCA positivity suggested vasculitis, the true etiology was bacterial endocarditis with secondary glomerular injury. Early steroid use likely worsened the infection. Clinicians must consider endocarditis in ANCA-positive patients with risk factors, defer immunosuppression until infection is ruled out, and pursue a tissue diagnosis when feasible.Renal biopsy showing cellular crescents with fibrin, global sclerosis, and a normal glomerulus, consistent with ANCA-associated crescentic glomerulopathy.