Culture-Negative Infective Endocarditis Presenting as ANCA-Positive Glomerulonephritis.

BACKGROUND Infective endocarditis is challenging to diagnose when blood cultures are negative, and its symptoms are nonspecific. Bartonella species are fastidious intracellular bacteria that can cause culture-negative endocarditis and immune-mediated complications such as glomerulonephritis, often mimicking ANCA-associated glomerulonephritis. CASE REPORT A 25-year-old man who underwent a Ross procedure for congenital aortic stenosis performed in childhood presented with 1 month of night sweats, abdominal pain, and intermittent nausea. Initial evaluation revealed elevated creatinine, pancytopenia, proteinuria, microscopic hematuria, and splenomegaly. Serologies showed positive c-ANCA and elevated PR3 antibodies. A presumed diagnosis of ANCA-associated glomerulonephritis led to initiation of high-dose steroids and planned rituximab. Kidney biopsy demonstrated crescentic glomerulonephritis with C3-dominant immune complex deposition, atypical for pauci-immune disease, prompting discontinuation of steroids. Further infectious workup revealed high Bartonella henselae IgM and IgG titers. The patient met the criteria for possible infective endocarditis under the 2023 Duke-ISCVID criteria. Transesophageal echocardiography showed valvular abnormalities without definite vegetations, although only a prior transthoracic echocardiogram was available for comparison. The patient was treated with doxycycline and rifampin for 12 weeks, resulting in clinical and laboratory improvement. CONCLUSIONS This case demonstrates that Bartonella endocarditis can closely mimic ANCA-associated glomerulonephritis and rapidly progressive glomerulonephritis. In patients with systemic symptoms, positive ANCA serologies, and negative cultures, occult infection must be a key consideration, especially in patients at high risk for infective endocarditis. A careful exposure history, including pets, travel, and dental procedures, can provide crucial diagnostic clues. Early recognition and targeted antimicrobial therapy are essential to prevent irreversible organ damage and avoid inappropriate immunosuppression.