Repeat kidney biopsy reveals new-onset IgA nephropathy in a patient with ANCA-associated vasculitis.

Microscopic polyangiitis (MPA) 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 rare. We report the case of an 81-year-old woman diagnosed with renal-limited AAV nine years earlier. Initial presentation was characterized by elevated myeloperoxidase (MPO)-ANCA levels and crescentic glomerulonephritis. Remission was achieved with prednisolone, and her serum creatinine level remained stable at approximately 0.9 mg/dL with low-dose maintenance therapy. Over the course of follow-up, she 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, the MPO-ANCA level had been gradually increasing, and she developed proteinuria (3.44 g/gCr), microscopic hematuria (20-29/high-power field), and an elevated serum creatinine level (1.28 mg/dL), suggestive of a relapse of AAV presenting as rapidly progressive glomerulonephritis. However, a repeat kidney biopsy revealed new-onset IgA nephropathy. This case highlights the new-onset development of IgA nephropathy during long-term maintenance therapy for MPA. It underscores the importance of considering alternative glomerular pathologies and performing repeat kidney biopsy in patients with established AAV who present with clinical deterioration. Given the rarity of coexisting AAV and IgA nephropathy, further case accumulation and investigation are warranted to better understand their potential association.