PAH EXTRACTION AND ESTIMATION OF PLASMA-FLOW IN DISEASED HUMAN KIDNEYS

We have analyzed the efficiency with which p-amino-hippuric acid (PAH) is extracted (E(PAH)) by patients with healthy kidneys (n = 13) or kidneys damaged by chronic cyclosporin nephropathy (n = 21) or primary glomerulopathy (n = 12); respective values (means +/- SE) for E(PAH) were 0.87 +/- 0.03, 0.77 +/- 0.03, and 0.69 +/- 0.04. Judged by a I-131-hippuran-to-PAH clearance ratio of 0.75 +/- 0.05, extraction ratio of hippuran was less efficient than E(PAH) in three glomerulopathic patients. A direct relationship was defined between E(PAH) and glomerular filtration rate (GFR) (r = 0.54) or calculated efferent oncotic pressure (II(E); r = 0.41, P < 0.01). Curve fitting by means of quadratic spline functions revealed GFR and II(E) to be additive in predicting E(PAH) (R2 = 0.45). Linear model prediction methods and a sample reuse technique failed to predict E(PAH) reliably from GFR and preglomerular oncotic pressure (II(A)); however, 95% prediction intervals exceed 0.30 E(PAH) units in width. We conclude that oncotic pressure (presumably reflecting albumin concentration) along with GFR is predictive of E(PAH) depression in humans with chronic renal disease. However, even sophisticated curve-fitting techniques are too imprecise for accurate prediction of E(PAH) in a given individual. We submit that renal venous sampling to determine E(PAH) continues to be necessary for the accurate determination of the rate of plasma flow in the injured human kidney.