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

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 (COPalb) underestimated the measured COP (COPm) by mean of 2.0 mmHg (p less than 0.002), with correlation coefficient r = 0.39(n/s). Similarly, the COPalb underestimated COPm by a mean of 5.7 mmHg (p less than 0.001) in the critically ill patient group; r = 0.38 (p less than 0.02). Furthermore, in the patients receiving plasma volume replacement serum albumin concentration fell by 13.1% (p less than 0.001) whilst COPm increased by 11.5% (p less than 0.002). In the normal subjects COP calculated from total protein concentration (COPtp) underestimated the COPm by 1.5 mmHg (p less than 0.02) with r = 0.65 (p less than 0.01). Conversely, in the patient samples, mean COPtp overestimated COPm by 3.5 mmHg (p less than 0.001) with r = 0.73 (p less than 0.002). We conclude that COPalb is an inadequate estimate of COPm particularly in patients where its use may have important clinical consequences. COPtp provides a reasonable estimate of COPm in normal subjects but in patients samples, where albumin: globulin ratio is low COPtp overestimates substantially in many cases. We advocate the direct measurement of COP in critically-ill patients.