Constant infusion clearance is an inappropriate method for accurate assessment of an impaired glomerular filtration rate

Recently renewed interest has been focused on constant infusionclearance to assess GFR accurately. In this study we comparedGFR and ERPF calculated from the constant infusion method (CIM= I x V/P) with that calculated from the standard method (StM= UxV/P), in 100 patients with renal disease who were subdividedin four groups according to their GFR-StM (<30;30–60;60–90; >90 ml/min). After a priming dose, a constantinfusion of ¹²⁵I-iothalamate (=GFR) and ¹³¹I-hippurate (=ERPF)was started at 9 a.m. The infusion rates were individually adjustedto the GFR which was approximated from the serum creatinineconcentration. After a 90-min equilibration period, GFR-StMand ERPF-StM were determined for two 2-h periods. These valueswere compared with GFR-CIM and ERPF-CIM calculated from theplasma concentration of the respective tracers at the end ofeach 2-h period (=210 and 330 min). In the patient group with GFR-StM <30 ml/min, the ¹²⁵I-iothalamateplasma concentration increased progressively over time. Consequently,average GFR-CIM at 210 min (34.2, SE±2.1 ml/min) washigher than the GFR-CIM at 330 min (31.9, SE±2.0 ml/min;P<0.001). In addition both values were significantly higherthan the corresponding GFR-StM values (18.1±2.4 and 15.3± 1.6 ml/min respectively). In the two patient groupswith GFR-StM > 60 ml/min, the ¹²⁵I-iothalamate plasma concentrationdecreased progressively over time. Consequently in both groupsGFR-CIM at 210 min was significantly lower than GFR-CIM at 330min. In all groups the ¹³¹I-hippurate plasma concentrationswere constant during the second 2-h period. Because constantplasma concentrations of ¹²⁵I-iothalamate are not obtained duringa 330 min period and GFR-CIM overestimates GFR-StM in patientswith low GFRs, we conclude that the constant infusion techniqueis not a valid method to assess GFR accurately if renal functionis impaired.