The effective oncotic pressure of dextran

Measurement of plasma volume before and after the acute infusion of commercial dextran revealed that all brands tested were hyperoncotic by a factor of approximately 2. By mixing 6% dextran with 0.9% saline in 1:1 proportion a 3% dextran solution was obtained which was nearly isoncotic in our dogs. The most likely explanation for the discrepancy is that the 6% solution contains a high proportion of molecules of lower molecular weight resulting in a greater oncotic pressure than predicted on the basis of the average molecular weight. In addition, this leads to a rapid loss of this solute from the blood. Clinicians and researchers should be aware of the possibility that the dextran preparation they are using is not truly isoncotic.