[Constant-infusion technique of inulin clearance without urine collection]

Inulin clearance is accepted as the gold standard for estimating the glomerular filtration rate (GFR). However, the method of this examination is troublesome and infants need difficult bladder catheterization. The existence of residual urine results in an inaccurate estimation of GFR and the proceduse requires a large amount of transfusion. In the plasma infusion method, inulin reaches an equilibrium in which the inulin urinary excretion rate is equal to the infusion rate, and urine collection is unnecessary. We estimated GFR in 21 children using both the plasma infusion method and renal infusion method. In the renal infusion method, the loading infusion of 1% inulin was administered over 30 minutes at the dose of 5 mL/kg body weight, followed by maintenance infusion at the constant speed (mL/hour) of 1.5 x estimated GFR (mL/min/1.73 m2) x body surface area (m2)/ 1.73. Three 30-minute urine collections were performed and the plasma inulin levels were measured to estimate GFR. In the plasma infusion method, maintenance infusion was conducted at the speed (mL/hour) of 0.6 x estimated GFR (mL/min/1.73 m2) x body surface area (m2)/1.73. The mean plasma inulin concentrations at 8, 9 and 10 hours were examined and GFR was calculated with the infusion rate. The GFRs for the renal infusion methods (Cin) and plasma infusion methods (e-Cin) were 91.90 +/- 39.61 and 95.33 +/- 38.08 mL/min/1.73 m2, respectively. The values for Cin and e-Cin showed good linear correlation (R2 = 0.81). The value of e-Cin/Cin was 1.069 +/- 0.172 and the mean e-Cin value was only 7% higher than that of Cin. We believe that GFR estimated by the constant infusion method shows a value approximating that estimated by the standard method. This technique is noninvasive for infants and the GFR of children who have vesicoureteral reflux or residual urine in the bladder can be estimated. The method does not need a large amount of transfusion and is suitable for children with heart failure. We believe that this method is clinically useful.