Measurement of colloid osmotic pressure in body fluids: Errors caused by preheparinized glass capillaries and by CO₂ loss

Abstract

The effect on colloid osmotic pressure (COP) of heparinizing body fluids was estimated with a low compliant osmometer, using Diaflo PM-30 or PM-10 membranes (Amicon, Lexington, Mass., USA). It was found that collecting and storing samples in preheparinized glass capillaries may increase COP by up to 4.0 mmHg. Measurements on heparin and protein solutions, separately and mixed, show that these macromolecules have a mutually potentiating effect on COP, probably by excluding part of the water as distribution space for the other molecular species. While this ‘heparin error’ varies among various types and batches of capillaries (Vitrex, Modulohm I/S), the content of heparin in some batches appears to be two to three times greater than the declared minimum. Alternatively, the excess COP may result from addition of other water-soluble macromolecules in the heparinization process. Even if some batches do not give appreciable error, we recommend to avoid preheparinized capillaries for measurement of COP. Both defibrination, and the amount of heparin needed to anticoagulate macro blood samples, have insignificant effect on COP. Loss of CO₂ by diffusion from separated plasma may increase pH towards 9.5. Concomitantly, COP increased by 2.1 mmHg per pH-unit. If plasma or serum samples are capped within some minutes after separation, they may be stored for weeks at 4 °C in polyethylene tubes without appreciable change of COP.

Key words:

• anticoagulants
• blood plasma
• Donnan osmotic pressure
• interstitial fluid
• lymph
• mucopolysaccharides
• plasma proteins