FIGURE 1. The dotted line represents the relation between plasma concentration (P) and the urine excretion rate (UV) ofa substance neither reabsorbed nor excreted by the renal tubules when the plasma concentration is increased by infusion. The continuous line represents the relationship during the infusion of phosphate in a subject with the same glomerular filtration rate (GFR). The tubular maximum reabsorptive capacity (TmP) is the distance A on the y axis between the dotted line and the linear portion of the continuous curve. Since the slope of the line (UV/P) is GFR, TmP/GFR is the intercept ofthe linear part ofthe curve with the x axis.(Reproducedfrom Reference 7 by permission of Butterworth-Heinemann Ltd and the editors.) linear relationship between the increase of plasma phosphate and the rate of phosphate excretion. TmP/GFR is the intercept of the linear part of the curve with the plasma concentration axis.

Bijvoet's infusion technique is clearly impracticable for routine use. However, Bijvoet, Morgan and Fourman/were able to use the relationships in the infusion data between plasma phosphate concentration, the phosphate/creatinine clearance ratio and TmP/GFR to predict TmP/GFR from fasting urine and