Renal Clearance Investigations with 51Cr-Edta and 125I-Hippuran

Renal clearance rates for 51Cr-EDTA, inulin, 125I-hippuran and PAH have been determined simultaneously in 25 patients using a constant infusion technique. By measuring the radioactivity in whole blood and calculating the plasma activity by using the haematocrit value, the clearance determinations for 51Cr-EDTA can be somewhat simplified, The clearance of 51Cr-EDTA was slightly lower than the clearanca of inulin. In order to obtain a correct value for the glomerular filtration rate, the clearance value for 51Cr-EDTA should be multiplied by a correction factor of 1.041 when calculating on the basis of a whole blood activity determination and 1.099 when the calculation is based on a plasma activity determination. The clearance of 125I-hippuran averaged 79% of the clearance value for PAH. The probable cause of this difference is that some of the 125I-hippuran is rather firmly bound to plasma protein and therefore not available for renal excretion. If 125I-hippuran is used as a substitute for PAH in the determination of the effective renal plasma flow, the clearance value for 125I-hippuran must be multiplied by a correction factor of 1.268. The extraction ratio for 125I-hippuran was also lower than for PAH. A PAH extraction ratio of 0.90 ± 0.05 corresponds to 0.74 ± 0.12 for 125I-hippuran.