A 38-year-old woman with systemic lupus erythematosus (SLE) is referred to the nephrology clinic for evaluation of lupus nephritis. Her serum creatinine is 1.8 mg/dL, blood urea nitrogen is 35 mg/dL, and serum sodium is 136 mEq/L. A 24-hour urine collection shows a total protein excretion of 4.2 g/day and urine creatinine of 800 mg/day. Her body weight is 60 kg. Using the Cockcroft-Gault equation (assuming age 38, female, non-obese), her estimated GFR is approximately 32 mL/min/1.73 m². The nephrologist orders a renal biopsy, which confirms Class IV lupus nephritis with 60% glomerular involvement. Before initiating immunosuppressive therapy, the team needs to assess baseline renal function. Which of the following statements about renal clearance measurement in this patient is most accurate?