A 24-year-old woman presents with high fever and arthralgia for half a month, edema for 10 days, blood pressure (BP) 150/90 mmHg. Over the past 7 days, she developed apathy and hypophonia. For the last 3 days, her urine output was 200-400 ml/day, with urinary protein (+++) and urinary red blood cells (RBCs) 15-20/HP. Hemoglobin (Hb) 80 g/L, C3 0.42 g/L, albumin (Alb) 23 g/L, serum creatinine (Scr) 400 μmol/L. Which test is LEAST helpful for diagnosis?