Comparison of iothalamate clearance methods for measuring GFR

Study Objective. To evaluate the bias and precision of three methods of measuring glomerular filtration rate (GFR) relative to a standard method. Design. Prospective, outpatient study. Setting. University-affiliated general clinical research center. Patients. Twenty-six patients with various degrees of renal function (GFR range 25-151 ml/min/1.73 m(2)). Interventions. Each patient received iothalamate twice during the study visit, first as a bolus injection and then as a priming dose followed by a constant-rate infusion for 2.5 hours. Measurements and Main Results. Plasma (Clp(IVB)) and renal clearances (Clr(IVB)) after bolus injection and plasma clearance during constant-rate infusion (Clp(INF)) were compared with standard renal clearance during constant-rate infusion (Clr(INF)). All three measures were highly correlated with Clr(INF) (r>0.90, p<0.001). The mean Clr(IVB) was not significantly different from Clr(INF) (106.3 +/- 30.4 vs 104.2 +/- 28.5 ml/min/1.73 m(2)) and provided a precise (8.8%, 95% CI 6.5-11.1%) and unbiased measure of GFR. Both Clp(IVB) and Clp(INF) were positively biased; values exceeded Clr(INF) by 11.8 +/- 11.1 (p=0.0001) and 10.5 +/- 12.5 ml/min/1.73 m(2) (p=0.0003), respectively. Use of a nonrenal correction factor of 9.8 and 10.5 ml/min/1.73 m(2) for infusion and bolus plasma clearance values, respectively, eliminated bias and improved the precision of these methods. Conclusions. Iothalamate renal clearance after bolus injection is a simple, accurate, and precise measurement of GFR and may be a useful alternative to the standard infusion method in clinical investigations. The corrected plasma clearance provides a simple index of GFR for clinical practice.