Rapidly progressive glomerulonephritis in rheumatoid arthritis: an early presentation of ANCA-associated vasculitis.

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis is a multisystem autoimmune disease that may present diagnostic challenges, particularly in patients with coexisting autoimmune conditions. Although the coexistence of rheumatoid arthritis and ANCA-associated vasculitis is recognised, vasculitis more commonly develops several years after rheumatoid arthritis onset. We describe a 68-year-old man who developed rapidly progressive glomerulonephritis due to myeloperoxidase (MPO)-ANCA-associated microscopic polyangiitis within 6 months of a diagnosis of seropositive rheumatoid arthritis. The diagnosis was established following an acute deterioration in renal function, positive MPO-ANCA serology and renal biopsy demonstrating pauci-immune necrotising crescentic glomerulonephritis. This case emphasises the importance of considering ANCA-associated vasculitis in patients with rheumatoid arthritis who develop acute kidney injury, even early in the disease course.