Isolated Acute Tubulointerstitial Nephritis in a Myeloperoxidase-ANCA-Positive Patient Without Crescentic Glomerulonephritis.

Kidney involvement in ANCA-associated vasculitis is usually recognized by its hallmark lesion, necrotizing crescentic glomerulonephritis, and clinicians naturally anchor their diagnostic expectations to this pattern. Rarely, however, the injury appears largely confined to the tubulointerstitium, with glomeruli that show no necrotizing or crescentic lesions, a presentation that is easily mistaken for drug-induced or infection-related interstitial nephritis. We describe an elderly woman who developed dialysis-requiring severe kidney failure soon after a brief febrile illness and was found to have strongly positive myeloperoxidase ANCA. Contrary to expectation, her kidney biopsy revealed active acute tubulointerstitial nephritis without any necrotizing or crescentic glomerular lesions, even though her urine contained dysmorphic erythrocytes, raising the possibility that glomerular injury was present but not captured in the sampled tissue. She recovered steadily with corticosteroid therapy and became dialysis-independent. This case serves as a reminder that ANCA-associated kidney disease can present without its classic glomerular signature, and that an early biopsy followed by timely immunosuppression can salvage kidney function in this unusual phenotype.