Background: ANCA-associated vasculitis (AAV) typically presents with pauci-immune crescentic glomerulonephritis. Isolated tubulointerstitial nephritis (TIN) as an initial manifestation is rare and may delay diagnosis.
Case Report: A 39-year-old man developed acute kidney injury, with a serum creatinine level of 5.11 mg/dL, after exposure to amoxicillin–clavulanate. Initial renal biopsy revealed acute TIN with eosinophilic infiltration and no crescents. Despite corticosteroid therapy and hemodialysis, renal function did not improve. Two weeks later, the patient developed massive hemoptysis and tested positive for c-ANCA. Repeat biopsy demonstrated pauci-immune crescentic glomerulonephritis, with crescents in 18 of 20 glomeruli. Intensive immunosuppressive therapy was initiated; however, the patient remained dialysis-dependent.
Conclusion: Steroid-resistant TIN may represent an early manifestation of AAV. Although acute TIN is most often drug-related, it should be kept in mind that it may be the first sign of various systemic diseases. Therefore, rebiopsy should not be delayed in treatment-resistant cases.