PEDIATRICS Vol. 48 No. 3 September 1971, pp. 377-392
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EXTERNAL MONITORING AND PLASMA DISAPPEARANCE FOR THE DETERMINATION OF RENAL FUNCTION: COMPARISON OF EFFECTIVE RENAL PLASMA FLOW AND GLOMERULAR FILTRATION RATE

Melvin L. Cohen M.D.1, Jivan K. Patel M.D.1, and David L. Baxter R.T.1

1 Departments of Pediatrics and Nuclear Medicine, St. Joseph's Hospital and Medical Center, Phoenix, Arizona

In 40 patients effective renal plasma flow (ERPF) was measured by analyzing mathematically the plasma disappearance curve and the precordial external monitor disappearance curve of Na hippuran I131 simultaneously after a single injection of 30 µCu. The monitor curve is variably more shallow than the actual plasma curve. A mathematical interpolation is described using two plasma samples to correct the external monitor curve to approximate the entire plasma curve. ERPF determined by using the corrected monitor curve correlates well (r = .998) with the ERPF determined from the actual plasma curve and gives a small scatter (S.E. of estimate 11.1). The terminal exponential slope of plasma disappearance of Na hippuran I131 is apparent before 30 minutes and ERPF can then be obtained in a 40-minute study.

Glomerular filtration rates (GFR) were performed in 34 patients using 10 µsCu of Na iothalamate I125, comparing values derived by blood and urine sampling with those derived by plasma disappearance.

The latter method gave consistently falsely high values. The terminal exponential slope of the plasma disappearance curve for this isotope was not apparent before 80 or 90 minutes. Determination of GFR by plasma disappearance alone using Na iothalamate I125 is invalid in studies lasting up to 100 minutes after a single injection.

Submitted on January 18, 1971
Accepted on April 12, 1971