A 35-year-old woman with systemic lupus erythematosus presents with proteinuria (3.2 g/day) and serum creatinine 1.4 mg/dL. Her serum albumin is 2.8 g/dL and hemoglobin is 9.2 g/dL. To differentiate between lupus nephritis with active glomerular disease versus chronic sclerotic changes, which investigation is most specific for assessing current glomerular filtration function?