RELATIONSHIPS AMONG PROTEIN AND ALBUMIN CONCENTRATIONS AND ONCOTIC PRESSURE IN NEPHROTIC PLASMA

We determined oncotic pressure (pi) by membrane osmometry and assayed total protein (TP) and albumin (Alb) concentrations in plasma of 102 n ephrotic subjects and 27 healthy controls. All three quantities were m arkedly depressed in the nephrotic group. When plasma was serially dil uted and concentrated, nephrotic but not control plasma also exhibited a highly variable change point in the nonlinear relationship between TP or Alb and pi. Absent a unique change point, we developed quadratic models which incorporated TP, Alb, and (TP x Alb) to prospectively pr edict pi in unperturbed plasma. The ability of the most successful qua dratic model to predict pi in afferent or efferent arteriolar plasma w as limited; the prediction errors reached 10 mmHg in nephrotic and 6 m mHg in control subjects. The nephrotic model coefficients also differe d significantly from control and pointed to an important influence of nonalbumin proteins on pi in nephrotic plasma. Investigation of the in trinsic membrane properties of diseased glomerular capillary walls req uires precise knowledge of pi. For this purpose we recommend that pi b e directly determined by membrane osmometry rather than calculated fro m protein concentration(s).