Precision of glomerular filtration rate determinations for long-term slope calculations is improved by simultaneous infusion of 125I-iothalamate and 131I-hippuran.

In studies on the progression of chronic renal failure the measurement of GFR must be very reliable. Sequential determination of GFR using the renal clearances of exogenous tracers such as inulin or iothalamate is the most accepted method. However, because of inaccuracies in urine collection, intratest variation, and thus intertest variation, of these clearances is considerable. This has a negative impact on the precision of long-term slope estimations. A previously described method of GFR determination on the basis of simultaneous infusion of 131I-hippuran and 125I-iothalamate corrects for inaccurate urine collection. To study whether this correction method improves the precision of the GFR slope measurement, this study analyzed longitudinal GFR data obtained in 71 patients with renal disease during a follow-up of 84 to 180 wk (477 renal function studies). All GFR were calculated by using both the standard renal clearance method and the correction method. The intratest and intertest coefficient of variation was significantly smaller for the correction method compared with the standard method (1.93 +/- 0.20 versus 8.48 +/- 1.66% P < 0.0005; and 2.88 +/- 0.32 versus 5.12 +/- 0.66%, P < 0.005, respectively). As a result, the precision of the GFR slope estimation was significantly better with the correction method compared with the standard method (error of the slope, 1.63 +/- 1.09 versus 2.35 +/- 2.36 mL/min per yr, P < 0.01). This improvement in precision of the slope by using the correction method reduces the necessary sample size needed to detect a GFR slope difference between interventions to about 30% of that needed when using the standard method. It is concluded that the precision of GFR measurements is improved by using correction for inaccurate urine collection with concomitant 131I-hippuran clearance.