Factors influencing the determination of colloid osmotic pressure of plasma

The present experiment was performed to examine the influence of pH and temperature of plasma samples on the determination of colloid osmotic pressure (COP). A linear increase in COP was observed when pH of the plasma increased. A significant linear correlation was also observed between COP and the temperature of the samples. The latter relation was explainable by van't Hoff's thermodynamic law, but the former could not always be interpreted by the Donnan's membrane equilibrium. These results suggested the importance of a correction for pH and temperature of the sample when the measurement was made in a condition different from pH 7.4 and 37 degrees C. An equation to predict COP was derived empirically from samples with a wide range of the plasma protein concentration (C) obtained by diluting a pooled rat plasma without altering the albumin-to-globulin ratio (A/G). A possible error in calculating COP was tested in samples with various levels of A/G. There existed a discrepancy of COP by approximately 10% between the measured value and the calculated one even if A/G was identical to the level from which the equation was derived. The difference of the two values became larger when the difference between the A/G for the equation and that of the test sample was higher. From the above experiment we can conclude that a reliable value of COP can be obtained only by a direct measurement of COP.