Recently renewed interest has been focused on constant infusion clearance to assess GFR accurately. In this study we compared GFR and ERPF calculated from the constant infusion method (CIM = I × V/P) with that calculated from the standard method (StM = U×V/P), in 100 patients with renal disease who were subdivided in four groups according to their GFR-StM (<30;30–60; 60–90; >90 ml/min). After a priming dose, a constant infusion of 125I-iothalamate (=GFR) and 131I-hippurate (=ERPF) was started at 9 a.m. The infusion rates were individually adjusted to the GFR which was approximated from the serum creatinine concentration. After a 90-min equilibration period, GFR-StM and ERPF-StM were determined for two 2-h periods. These values were compared with GFR-CIM and ERPF-CIM calculated from the plasma concentration of the respective tracers at the end of each 2-h period (=210 and 330 min). In the patient group with GFR-StM <30 ml/min, the 125I-iothalamate plasma concentration increased progressively over time. Consequently, average GFR-CIM at 210 min (34.2, SE±2.1 ml/min) was higher than the GFR-CIM at 330 min (31.9, SE±2.0 ml/min; P<0.001). In addition both values were significantly higher than the corresponding GFR-StM values (18.1±2.4 and 15.3 ± 1.6 ml/min respectively). In the two patient groups with GFR-StM > 60 ml/min, the 125I-iothalamate plasma concentration decreased progressively over time. Consequently in both groups GFR-CIM at 210 min was significantly lower than GFR-CIM at 330 min. In all groups the 131I-hippurate plasma concentrations were constant during the second 2-h period. Because constant plasma concentrations of 125I-iothalamate are not obtained during a 330 min period and GFR-CIM overestimates GFR-StM in patients with low GFRs, we conclude that the constant infusion technique is not a valid method to assess GFR accurately if renal function is impaired.
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Guldener et al. (1995) studied this question.