Prediction of colloid osmotic pressure in renal patients

Background: Colloid osmotic pressure (COP) plays a major role in transcapil lary fluid shift, including in the glomerular capillary. However, COP is ge nerally estimated by quadratic equations derived from total plasma protein and/or albumin concentrations. The aim of this study was to assess the accu racy of such equations, and to determine the potential role of liver-derive d non-albumin proteins in the maintenance of COP, especially in patients pr esenting a nephrotic syndrome. Methods: COP was directly assessed with an o smometer in 170 patients (347 samples), and the results compared with calcu lated COP, using 4 previously published formulas [Brenner 1972, Canaan-Kuhl 1993, Landis-Pappenheimer 1963, Navar 1977]. Results: The 4 calculated COP values were strongly correlated with measured COP (range r = 0.88 - 0.96). However, in absolute terms, measured COP differed significantly from each of the 4 calculated mean values of COP (p < 0.001). Fibrinogen exerted per se a weak oncotic effect as measured in vitro. However, fibrinogen was high ly related to albumin and presumably reflected the oncotic effect of other liver-derived non-albumin proteins. Inclusion of albumin and fibrinogen in a linear model provided an excellent fit for predicted COP with a highly si gnificant correlation (r = 0.96, p < 0.001) over a wide range of COP values . The predicted equation was: COP(mmHg) = 6.89 x (albumin + fibrinogen) (g/ dl) - 5.68. Conclusion: None of the 4 most commonly used formulas correctly predicted COP, and direct measurement of COP is still preferable for resea rch studies. The introduction of fibrinogen into the formula estimating COP leads to higher accuracy, and therefore represents a more convenient model for routine evaluation.