A COMPARISON OF SINGLE BLOOD-SAMPLE AND MULTIPLE BLOOD-SAMPLE METHODSTO MEASURE GFR IN A PEDIATRIC POPULATION

Several single-sample, Cr-51 EDTA based methods of estimating glomerul ar filtration rate in children have been proposed. The authors retrosp ectively reviewed our experience in 467 pediatric patients using Tc-99 m DTPA and 4 ultrafiltered serum samples drawn 2-4 hours postinjection . The authors used the 2-hour specimens for the single-sample method. Regression analysis of the 2 techniques revealed significant scatter. Mathematically, it was shown that the error of the single-sample metho d was minimized when the clearance t1/2 equalled the sampling time x 0 .693. This model predicted a 300% error when the t1/2 was varied to it s observed extremes. The observed error of the single-sample method fi t this theoretical prediction very well. Analysis of a two-sample meth od versus the four-sample method showed no improvement in results. In a subset of 18 unfiltered plasma samples, the glomerular filtration ra te was falsely low in 12 patients. The authors conclude that multiple blood samples and serum ultrafiltration are required to accurately mea sure glomerular filtration rate when using Tc-99m DTPA.