Modified plasma clearance technique using nonradioactive iothalamate for measuring GFR

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Glomerular filtration rate (GFR) is believed to be the overall index of renal function in health and disease. And the renal clearance of inulin (C In ) obtained during constant intravenous infusion has long been accepted as the gold standard of GFR measurement. Because of a number of technical difficulties inherent in the assay of inulin concentration in urine and plasma, its usefulness in clinical practice is limited. Iothalamate, a urographie contrast medium which behaves and is excreted in a fashion similar to inulin, has been accepted as a good filtration marker [1–3]. The renal clearance of iothalamate is not much more simplified than that of inulin. But its measurement is used instead of inulin in the assessment of GFR. Whether inulin or iothalamate is used, the measurement of renal clearance requires proper collection and measurement of an accurate short-timed urine specimen. Therefore improper urine collection sometimes leads to error in GFR measurement. To overcome this disadvantage, GFR is alternatively determined as the total plasma clearances calculated from serial measurement of plasma concentrations after a single rapid intravenous injection, where urine collection is not necessary [4, 5]. The total plasma clearance should be equal to glomerular filtration rate, if the compounds would be excreted solely by glomerular filtration without tubular secretion or reabsorption. The conventional method of the total plasma clearances, however, usually needs multiple blood samples to calculate from the slope of the plasma disappearance curve. The plasma disappearance curve is generally separated into two compartments: the early phase reflects the distribution of the marker and the late phase reflects the excretion of the marker. The latter is closely related with the clearance rate [6]. So, the late phase of the plasma disappearance curve would offer a simple and good approximation of GFR, and it would conquer the disadvantages of the renal clearances and total plasma clearances, that is, timed urine collections and multiple blood sampling, respectively. The aim of this investigation was to study whether GFR could be estimated by the modified plasma clearance technique to calculate only from the late phase of the plasma decline at 60, 90, and 120 minutes, without urine collection, when iothalamate was used as filtration marker and high-performance liquid chromatography (HPLC) system as the analyzing method.