Discordant ANCA Positivity in Immune Complex Glomerulonephritis Secondary to Bartonella henselae Prosthetic Valve Endocarditis.

Bartonella henselae is an intracellular bacterium which classically causes cat scratch disease as well as culture-negative endocarditis. We report a man in his 60s, with a mechanical aortic valve replacement who presented with a 6-month fever of unknown origin (FUO), splenomegaly and acute kidney injury. Investigations showed discordant ANCA serologies, screening anti-MPO positivity followed by confirmatory anti-PR3 positivity, raising concern for ANCA-associated vasculitis (AAV). However, renal biopsy demonstrated immune complex deposition, inconsistent with the pauci-immune glomerulonephritis typical of AAV. Subsequent investigations revealed positive Bartonella serology and PCR. Transoesophageal echocardiography confirmed the diagnosis of B. henselae prosthetic valve endocarditis. He was treated with a prolonged course of rifampicin and doxycycline which led to clinical improvement. This case illustrates how B. henselae can mimic AAV, with discordant ANCA results contributing to diagnostic uncertainty, and reinforces the need to consider alternative causes of ANCA positivity.