A 20-year-old female presents with pitting edema of the eyelids and both lower extremities, accompanied by oliguria for the past month. Laboratory tests show: serum albumin (Alb) 27g/L, A/G ratio 0.8, creatinine 133μmol/L, urinary protein (++++), 24-hour urinary protein quantification 5g, ANA titer 1:640, and anti-RNP antibodies. Which test is most significant for assessing disease activity?