Measurement of Tc-99m DTPA serum clearance for estimating glomerular filtration rate in children with cancer.

Clearance of the radiopharmaceutical Tc-99m DTPA estimated from blood samples with no urine collection can provide a reliable estimate of glomerular filtration rate (GFR) in adults, but has not been well studied in children. The disposition of Tc-99m DTPA was determined in 17 children with cancer, and the influence of binding and study design on the estimates for serum clearance were evaluated. Nine blood samples were obtained over 6 hours in each patient, and serum was assayed for total and free Tc-99m. Free Tc-99m DTPA was determined by ultrafiltration. Estimates of clearance derived from a two-compartment model for ultrafiltrable Tc-99m DTPA were determined from all nine measured concentrations, and these results served as a reference value for GFR in each subject. Total Tc-99m DTPA concentrations also were best described by a two-compartment model, but the median total clearance (35 ml/min) was significantly (p < 0.01) lower than the ultrafiltrate clearance (58 ml/min). The effect of a simplified sampling schedule was assessed from clearance estimates based on a 3-point subset of the ultrafiltrable data. The median clearance of 69 ml/minute was significantly higher (p < 0.01) than the reference GFR. However with a correction factor included to account for the positive bias arising from the limited sampling schedule, the reference estimates for GFR were well predicted (r2 = 0.99) with no significant bias. Ultrafiltrable Tc-99m DTPA serum clearance using a limited sampling schedule can provide a consistent and clinically feasible estimate of glomerular filtration rate in children, but binding in serum and study design are potentially important confounding factors.