A 21-year-old female presents with a 6-month history of low-grade fever and 2 weeks of hematuria. Urine sediment shows RBCs covering the entire field/HP, with 80% being dysmorphic RBCs. Urinary protein (++). Rash on the face and limbs. Blood pressure 150/90 mmHg. Laboratory tests reveal: WBC count 3.0×10^9/L, plasma albumin 3.6 g/L, Scr 80 μmol/L, IgG 18.2 g/L, C3 0.58 g/L, ANA 1:640. The clinical diagnosis is systemic lupus erythematosus (SLE). Which test is most helpful for confirming lupus nephritis?