Measurement of GFR with a single intravenous injection of nonradioactive iothalamate

Rigorous assessment of the glomerular filtration rate (GFR) requires the measurement of the renal clearance of inulin, a fructose polymer that fulfills the criteria for an ideal filtration marker [1, 2]. However, the classical inulin clearance method is not suitable for routine clinical purposes, since it requires continuous intravenous infusion and, sometimes, bladder catheterization of the patients [3]. Simultaneous studies of renal inulin clearance and plasma 51 Cr-EDTA clearance have demonstrated that the plasma clearance of 51 Cr-EDTA is a reliable index of GFR [4, 5], with the advantage that it avoids the need for urine collection and continuous intravenous infusion of the tracer. This simple and reliable technique is now commonly used in clinical practice, but still maintains the drawbacks associated with the use of radiolabeled compounds, such as the need for special licensing, compliance with complicated procedures for handling, storage and disposal of contaminated wastes, and the exclusion of certain patients, such as pregnant women, from the examination. The development of simple methods for the determination of iothalamate [6–9] and of other x-ray tracers [10–13] allows the use of unlabeled compounds for the measurement of GFR. Again, the need for urine collection and continuous intravenous infusion [9, 11, 14] or subcutaneous injection mixed with epinephrine [15] in order to maintain constant plasma concentration of the tracer, make this technique impractical for routine clinical use. In the present study we describe a rapid, simple and reliable method for the evaluation of renal function by the measurement of plasma clearance of unlabeled iothalamate. We also validate the method by comparing the plasma and the renal clearances of iothalamate with the plasma clearance of 51 Cr-EDTA in kidney patients with different degrees of renal function.