Beyond the Infection: A Case of C3 Glomerulopathy Mimicking Postinfectious Glomerulonephritis

Introduction: C3 glomerulonephritis (C3GN) is a rare, non-inflammatory glomerular disease characterized by consumption of C3 complement in glomeruli with absence or minimal immunoglobulin deposits, confirmed through kidney biopsy. We present a case of nephritic syndrome due to C3GN, initially thought to be due to post-infectious GN (PIGN). Case Description: 51-year-old male presented with lower extremity cellulitis and new-onset painless hematuria for 4 days. Laboratory results showed acute renal injury with serum creatinine 7 mg/dL, blood urea 88 mg/dL, potassium 5.9 mEq/L and phosphorus 6.1 mg/dL. Urinalysis showed proteinuria (3+), moderate RBCs and 11 WBCs/high-power field with a sterile culture. Complement levels were low (C3 33 mg/dL, C4 25 mg/dL). Antistreptolysin O titer was elevated at 396.1 IU/mL. ANA, ANCA and anti-ds DNA were normal. Renal US showed echogenic kidneys. Initially treated with IV antibiotics for suspected PIGN, eventually requiring hemodialysis due to persistent oliguria and uremic symptoms. Renal biopsy showed focal necrotizing and crescentic glomerulonephritis with C3-dominant deposits (see Figure 1). These findings are more characteristic of C3GN than PIGN. He was started on methylprednisolone 800mg IV for 3 days followed by prednisone 1mg/kg/day as well as two doses of rituximab 800mg. Renal function recovered remarkably with eventual discontinuation of hemodialysis. Creatinine on discharge had returned to the baseline of 1.2 mg/dL. Discussion: We present a case where new-onset nephritic syndrome was initially thought to be secondary to PIGN. After kidney biopsy, the diagnosis of C3GN was made with subsequent immunosuppressive treatment leading to recovery. This highlights the difficulty in diagnosis of C3GN and subsequent treatment.