This method for estimating GFR was compared with the 125 I-iothalamate method in 14 children and young adults. The method has several advantages compared to conventional clearance methods. Fewer blood samples are needed, no precisely timed urine collections are necessary, and there is no radioactivity exposure. The risk of allergic reactions is low. In most cases, the infusion is started in late afternoon and continued overnight at home.
Blood is collected early the next morning.
This approach avoids disruption of the normal school day. Estimates of GFR using the endogenous marker, cystatin C, have been recently described. It is a 13.3 kD protein, which is eliminated from the circulation almost exclusively by glomerular filtration. Studies in adults have shown cystatin C to be a more sensitive marker of changes in GFR than serum creatinine. Beyond the first year of life, cystatin C concentration is constant, while that of creatinine increases steadily until adulthood.