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A comparison of single blood sample and multiple blood sample methods to measure GFR in a pediatric population
R J Boudreau1, R T Carpenter, B E Hasselquist
1Department of Radiology, University of Minnesota, Minneapolis.
Insights
Accurate pediatric glomerular filtration rate (GFR) measurement using Tc-99m DTPA requires multiple blood samples and serum ultrafiltration. Single-sample methods, even at 2 hours, show significant error and are unreliable for GFR estimation in children.
Area of Science:
- Pediatric Nephrology
- Nuclear Medicine
- Diagnostic Imaging
Background:
- Glomerular filtration rate (GFR) is a key indicator of kidney function in children.
- Several single-sample methods using Cr-51 EDTA exist for pediatric GFR estimation.
- Tc-99m DTPA is a common radiotracer used for GFR assessment.
Purpose of the Study:
- To evaluate the accuracy of a single-sample Tc-99m DTPA method for pediatric GFR estimation.
- To compare the accuracy of single-sample, two-sample, and four-sample methods.
- To determine the necessity of serum ultrafiltration for accurate Tc-99m DTPA GFR measurements.
Main Methods:
- Retrospective review of 467 pediatric patients undergoing Tc-99m DTPA renography.
- Collection of 4 ultrafiltered serum samples 2-4 hours postinjection.
- Comparison of a single-sample method (using 2-hour specimens) with a reference method.
- Mathematical modeling to predict error in single-sample GFR estimation.
Main Results:
- Significant scatter observed between the single-sample method and the reference technique.
- Mathematical model predicted up to 300% error in single-sample GFR estimation across observed extremes.
- Two-sample method offered no improvement over the four-sample method.
- Unfiltered plasma samples led to falsely low GFR in 12 out of 18 patients.
Conclusions:
- Multiple blood samples and serum ultrafiltration are essential for accurate pediatric GFR measurement with Tc-99m DTPA.
- Single-sample methods are unreliable and prone to significant error.
- Serum ultrafiltration is critical to avoid falsely low GFR values.
Abstract:
Several single-sample, Cr-51 EDTA based methods of estimating glomerular filtration rate in children have been proposed. The authors retrospectively reviewed our experience in 467 pediatric patients using Tc-99m 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 method was minimized when the clearance t 1/2 equalled the sampling time x 0.693. This model predicted a 300% error when the t 1/2 was varied to its observed extremes. The observed error of the single-sample method fit this theoretical prediction very well. Analysis of a two-sample method versus the four-sample method showed no improvement in results. In a subset of 18 unfiltered plasma samples, the glomerular filtration rate was falsely low in 12 patients. The authors conclude that multiple blood samples and serum ultrafiltration are required to accurately measure glomerular filtration rate when using Tc-99m DTPA.