A generic method for analysis of plasma concentrations .

The purpose of this NephEd contribution is to introduce a generic method for analyzing plasma concentrations ([]). The method is applicable to substances that are filtered by glomeruli, reabsorbed or secreted by tubules, and excreted in urine. The equality from which the method follows states that the filtration rate of substance is the sum of excretion and net reabsorption rates of (F = E + TR). If is completely ultrafilterable, F = GFR[], and [] = E/GFR + TR/GFR. If creatinine clearance (C) is substituted for GFR, E/C simplifies to [][cr]/[cr], which can be calculated from measurements in simultaneous aliquots of serum and urine. TR/C is then deduced as [] - E/C. The ratios E/C and TR/C - the of [] - quantify the amounts of excreted and reabsorbed per volume of filtrate. If [] is abnormal, the generic method identifies which of the determinants is creating the abnormality. If [] is normal despite disruptive circumstances, e.g., a reduced GFR, the method elucidates the preservation of normalcy. Herein, we develop these concepts and illustrate their practical utility.