COP is most commonly measured using a membrane transducer system (Fig 1). The method requires detection of flow across a semipermeable membrane from a closed reference chamber to the sample chamber. As fluid moves from the closed reference chamber into the sample the hydrostatic pressure in the reference chamber becomes progressively more negative, tending to counterbalance the COP of the sample. When the hydrostatic pressure in the reference chamber equals the osmotic pressure in the sample chamber, an equilibrium is reached. Using such devices, it typically takes less than 3 min to measure the COP of a sample of heparinized blood, plasma, or serum which may be as small as 200 pl. However, when samples are small, it is often necessary to inject several aliquots into the oncometer. This is because the initial high pressure gradient between the COP of the sample and the zero-pressure conditions of the reference chamber causes significant sample dilution. With several repeated injections of sample and as the hydrostatic pressure in the reference chamber approaches the COP of the sample, dilution becomes insignificant. Although COP measurement is easy, there are several potential artifacts which may lead to erroneous results: free hemoglobin in the sample due to hemolysis has a significant COP, liquid heparin reduces COP by dilution, excess dry heparin molecules contribute to COP, a reduction in pH reduces the negative charge on protein, and fibrinogen molecules contribute to COP.
Fig. 1 A membrane transducer oncometer. The COP is the negative pressure in the reference chamber which prevents net movement of salt and water across the semipermeable membrane.
By analogy with the ideal gas laws, van't Hoff stated that the COP of an ideal solution (infinitely dilute) is proportional to the concentration of the colloid ( Webb.19,9,2).
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