When Interstitial Nephritis Hides Vasculitis: A Case of ANCA-Associated Vasculitis

Introduction: We present a case of anti-myeloperoxidase (MPO) antibody-positive Antineutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis (AAV) presenting as subacute interstitial nephritis on renal biopsy. Case Description: The patient is a 75-year-old male with a history of pulmonary fibrosis, hypertension, dyslipidemia, and coronary artery disease who presented with worsening renal function following a recent hospitalization where he was diagnosed with new-onset polymyalgia rheumatica. Labs showed a serum creatinine of 1.5mg/dL (known baseline of 1.0), and urinalysis showed trace protein and 1+ hematuria. Acute kidney injury was initially presumed to be secondary to NSAIDs and was stopped. Since the hospital discharge, he also experienced worsening peripheral neuropathy-like symptoms and a new-onset right foot drop. He was seen a week later, and his serum creatinine had continued to rise and was 2.8 mg/dL. His urine protein-creatinine ratio was 688mg/g, and his symptoms remained unchanged. Urine microscopy was negative for dysmorphic RBCs and casts. Serologic studies returned and were notable for perinuclear-ANCA titer of 1:160, rheumatoid factor level of 56U/mL, and anti-MPO antibody level of 33U/mL. Renal biopsy revealed focal subacute tubulointerstitial nephritis with no evidence of crescents, negative immunofluorescence for IgA, IgM, IgG, C3, C1q, and fibrinogen and electron microscopy showed subtotal foot process effacement. He was treated with intravenous methylprednisolone daily and then transitioned to a prednisone taper. Additionally, he was given Rituximab 1000mg with plans for continuation. About 1 month later, his renal function and overall symptom burden improved, and the anti-MPO antibody titers became undetectable. Discussion: This case represents a rare presentation of AAV with interstitial nephritis and without classical crescentic glomerulonephritis. These patients are usually older and have more nonspecific symptoms.