What is the best method for measuring renal function in adults and children with cystic fibrosis?

To measure the glomerular filtration rate (GFR) in adults and children with cystic fibrosis (CF) using a radio-isotope technique as the gold standard and to compare this to serum creatinine based equations, serum cystatin C levels and tobramycin clearance, and to determine which method correlates most closely with measured GFR in this population.