Repeat Kidney Biopsy in Suspected Relapse of ANCA-Associated Vasculitis: A Case of De Novo IgAN

Introduction: Microscopic polyangiitis is a subtype of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). Although relapse during maintenance therapy is relatively common, the coexistence or subsequent development of other glomerular diseases, such as IgA nephropathy, is exceedingly rare. Case Description: An 81-year-old woman was diagnosed with renal-limited vasculitis 9 years earlier. At initial presentation, she had elevated myeloperoxidase (MPO)-ANCA levels and crescentic glomerulonephritis. Remission was achieved with prednisolone induction therapy. Serum creatinine remained stable at approximately 0.9 mg/dL, and remission was maintained with low-dose prednisolone. She subsequently experienced two episodes of MPO-ANCA elevation accompanied by microscopic hematuria, both of which responded to an increased dose of prednisolone. At the current presentation, she had proteinuria (3.44 g/gCr), microscopic hematuria (20–29/HPF), and elevated serum creatinine (1.28 mg/dL), raising suspicion of a relapse of AAV presenting as rapidly progressive glomerulonephritis. The prednisolone dose was increased, and avacopan and rituximab were initiated. However, a repeat kidney biopsy revealed de novo IgA nephropathy. Avacopan was discontinued, prednisolone was tapered, and an angiotensin II receptor blocker was introduced. Thereafter, the patient’s proteinuria improved markedly and kidney function remained stable. Discussion: This case illustrates the de novo development of IgA nephropathy during long-term maintenance therapy for microscopic polyangiitis. It underscores the importance of considering alternative or coexisting glomerular pathologies and highlights the critical role of repeat kidney biopsy in establishing an accurate diagnosis when clinical deterioration occurs in a patient with established AAV. Given the rarity of concurrent AAV and IgA nephropathy, further collection and analysis of similar cases are warranted to elucidate their potential association.