A new centrifugal filtration device for free drug separation.

A new centrifugal membrane filtration device (Syva Corporation) has been evaluated to determine its capabilities for separation of free phenytoin for analysis. The device is a test tube-size cylinder with two compartments separated by the membrane. In serum samples from 70 patients at the Hospital of the University of Pennsylvania, free phenytoin was prepared by the new device and by equilibrium dialysis. Levels were assayed by gas chromatography and enzyme immunoassay with good agreement at all phenytoin levels. Although pH has a significant effect on the binding of some drugs to serum proteins, phenytoin binding increased to only a small extent as pH was increased from 7.0 to 7.8 (85-88.5% bound). Centrifugal filtration with this device is a reliable, fast, and easy way to prepare free drug from serum and does not include the dilution artifact inherent in equilibrium dialysis.