The direct measurement of plasma colloid osmotic pressure is superior to colloid osmotic pressure derived from albumin or total protein

Abstract

Plasma colloid osmotic pressure (COP) has been calculated from both serum albumin concentration and plasma total protein concentration. These values have been compared to those measured directly using a membrane-transducer oncometer in a group of normal subjects, in a group of critically-ill patients with a variety of primary diagnoses and in a group of hypovolaemic patients before and after plasma volume replacement with 6% hydroxyethyl starch solution. In the normal samples, COP calculated from albumin (COP_alb) underestimated the measured COP (COP_m) by mean of 2.0 mmHg (p(0.002), with correlation coefficient r=0.39 (n/s). Similarly, the COP_alb underestimated COP_m by a mean of 5.7 mmHg (p(0.001) in the critically ill patient group; r=0.38 (p(0.02). Furthermore, in the patients receiving plasma volume replacement serum albumin concentration fell by 13.1% (p(0.001) whilst COP_m increased by 11.5% (p(0.002). In the normal subjects COP calculated from total protein concentration (COP_tp) underestimated the COP_m by 1.5 mmHg (p(0.02) with r=0.65 (p(0.01). Conversely, in the patient samples, mean COP_tp overestimated COP_m by 3.5 mmHg (p(0.001) with r=0.73 (p(0.002). We conclude that COP_alb is an inadequate estimate of COP_m particularly in patients where its use may have important clinical consequences. COP_tp provides a reasonable estimate of COP_m in normal subjects but in patients samples, where albumin: globulin ratio is low COP_tp overestimates substantially in many cases. We advocate the direct measurement of COP in critically-ill patients.