The analytical basis of membrane separation in determining plasma free drug concentrations is developed from consideration of the generalised ideal mass action and conservation laws applied to thermodynamic states of differing component volumes. The intuitively surprising theoretical independence of free drug concentration with changing fractional volume of protein predicts the equivalence affinal free drug concentration after uniform pre-dilution with that after equilibrium dialysis against the same volume ofbuffer. Similarly. the ultra filtrate free concentration of drugs in ideal plasma binding equilibrium is theoretically constant. regardless of the extent of reduction in fractional volume of binding protein during filtration.The pi(fa/ls in calculating free and bound fraction and concentration from equilibrium dialysis are reviewed and expressions presented to correct the major artefacts. The dilution behaviour of drugs is predicted from the normalised mass action model. Practical aspects of l'alidating ultrafiltration free drug levels using equilibrium dialysis are outlined.
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