The Measurement of Glomerular Filtration Rate and Effective Renal Plasma Flow in Man by Iothalamate ¹²⁵ I and Iodopyracet ¹³¹ I

In 21 patients, 90 simultaneous renal clearances of inulin and sodium iothalamate 125 I, and para-amino-hippuric acid (PAH) and iodopyracet 131 I were determined by a constant infusion technique. The activities of iothalamate 125 I and iodopyracet 131 I were determined simultaneously in a dual-channel scintillation counter. The concentrations of inulin and PAH were determined by standard chemical techniques. The ratio of iothalamate 125 I to inulin clearance ranged from 0.93 to 1.09 with a mean of 1.00. The ratio of iodopyracet 131 I to PAH clearance varied from 0.86 to 1.04 with a mean of 0.96. The chemical determination of PAH and inulin required approximately 8 hours of technician time as compared with 30 minutes by the radioactive technique. The determination of the renal clearances of iothalamate 125 I and iodopyracet 131 I is an accurate and time-saving technique for the measurement of glomerular filtration rate and effective renal plasma flow.