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 m2). 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 (ClpIVB) and renal clearances (ClrIVB) after bolus injection and plasma clearance during constant‐rate infusion (ClpINF) were compared with standard renal clearance during constant‐rate infusion (ClrINF). All three measures were highly correlated with ClrINF (r>0.90, p<0.001). The mean ClrIVB was not significantly different from ClrINF (106.3 ± 30.4 vs 104.2 ± 28.5 ml/min/1.73 m2) and provided a precise (8.8%, 95% CI 6.5–11.1%) and unbiased measure of GFR. Both ClpIVB and ClpINF were positively biased; values exceeded ClrINF by 11.8 ± 11.1 (p=0.0001) and 10.5 ± 12.5 ml/min/1.73 m2 (p=0.0003), respectively. Use of a nonrenal correction factor of 9.8 and 10.5 ml/min/1.73 m2 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.