Kidney biopsy in patients with antineutrophil cytoplasmic antibody-associated vasculitis with mild renal abnormality

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis is a systemic inflammation of small or medium blood vessels that includes microscopic polyangiitis. A diagnosis of ANCA-associated vasculitis can be aided by histological identification of vasculitis, and identification of renal impairment can help predict outcomes. However, kidney biopsy is not generally indicated in the absence of renal findings. We report two cases of ANCA-associated vasculitis diagnosed by kidney biopsy despite the absence of remarkable urinary abnormality and renal impairment. These patients had fever of unknown origin and were positive for myeloperoxidase (MPO)-ANCA but showed few findings that would suggest small-vessel vasculitis in the kidney. Nevertheless, kidney biopsies revealed small-vessel arteritis, necrotizing glomerulonephritis, and interstitial nephritis. Immunofluorescent antibody tests performed using samples of glomeruli were all negative, suggesting microscopic polyangiitis. Therefore, kidney biopsy may be useful in confirming the diagnosis, even if patients have completely normal urinary findings in the absence of other organ lesions.