Iodine-131 Hippuran for the estimation of renal plasma flow: requirements for radiochemical purity

Abstract

For many years iodide-131 Hippuran has been used as a tracer to measure effective renal plasma flow (ERPF). Because of the low renal clearance of free¹³¹-Iiodide and the inability to count it separately from¹³¹I-Hippuran, free¹³¹I-iodide will lower the calculated¹³¹I-Hippuran clearance, resulting in a lower estimated ERPF. This study was performed to establish the maximum allowable radiochemical impurity of free¹³¹I-iodide in¹³¹I-Hippuran preparations for ERPF measurements in continuous clearance studies. A known amount of¹²³I-iodide was added to the (¹³¹I-iodide-free)¹³¹I-Hippuran solution used for continuous infusion clearance studies in nine patients.¹²³I-iodide activity was used because it can be counted separately from¹³¹I-Hippuran in the infusion solutions and plasma samples while it behaves exactly like¹³¹I-iodide, so that the results obtained with¹²³I-iodide can be extrapolated to¹³¹I-iodide. After performing the clearance studies, the ERPF was calculated firstly with¹³¹I-Hippuran activity only (=true ERPF) and secondly including the free radioactive iodide activity (=false ERPF) in the clearance formula. As expected, if free¹³¹I-iodide is present in the infusion solution, its concentration in plasma will be highest at the end of the clearance study. The¹³¹I-iodide concentration in plasma relative to the¹³¹I-Hippuran concentration will be higher in patients with high ERPF values.¹³¹I-iodide in the infusion solution causes a fall in the ERPF as measured by the continuous infusion technique: 0.5%, 1% and 2% of free¹³¹I-iodide in the infusion solution result in a reduction in ERPF of about 1.5%, 3.5% and 7% respectively after 1.5 h and of 3.5%, 6.5% and 13% respectively after 5.5 h if ERPF is high. It is concluded that a maximum of 0.5% of free¹³¹I-iodide in¹³¹I-Hippuran preparations is permissible if ERPF is to be measured with an error of less than 5%.