From Pyelonephritis to Vasculitis: A Challenging Diagnosis of Renal-Limited Vasculitis Presenting As Acute-on-Chronic Renal Failure.

Renal-limited vasculitis (RLV) is a rare form of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis. The kidneys are the primary, and often the only, organs affected by this disease. A 66-year-old white male presented with hematuria and acute-on-chronic renal failure. Computed tomography (CT) of the abdomen and pelvis revealed left-sided pyelonephritis/cystitis. Urinalysis showed elevated white blood cell (WBC) and red blood cell (RBC) counts, with 1+ bacteria and 3+ proteinuria. Serum creatinine was 5.6 mmol/L, and estimated glomerular filtration rate (eGFR) was 10 mL/min/1.73 m². The patient was admitted with a diagnosis of pyelonephritis and treated with antibiotics. The patient showed no clinical or laboratory improvement with antibiotics. Urine culture was negative, and the urine protein-to-creatinine ratio indicated nephrotic-range proteinuria. Further tests, including an antinuclear antibody (ANA) panel, anti-cytoplasmic antineutrophil cytoplasmic antibodies (ANCA), and urine and serum electrophoresis were ordered. Anti-myeloperoxidase (MPO) antibody was positive. The patient subsequently underwent a kidney biopsy, which revealed pauci-immune crescentic glomerulonephritis, severe interstitial fibrosis, severe tubular atrophy, and arterionephrosclerosis. The patient was diagnosed with Myeloperoxidase (MPO)-associated RLV and was started on pulse dose steroids, prednisone 1 milligram per kilogram (mg/Kg) daily, and weekly Rituximab 375 milligrams per square meter (mg/m²). The patient received two doses of Rituximab while inpatient, with significant improvement in kidney function. This case highlights the importance of considering the possibility of underlying vasculitis in cases of refractory renal failure and proteinuria, even when initial symptoms, laboratory tests, and imaging suggest an infection.