Free phenytoin has been determined using micro-scale ultracentrifugation followed by analysis by EMIT. The effect of temperature on the determined free fraction was investigated and the ultracentrifugation procedure validated against ultrafiltration. Ultracentrifugation gave free fractions which were on average 16% lower than those obtained using ultrafiltration, but correlation was good, as was the correlation with measurements of total phenytoin (r=0·90). Micro-scale ultracentrifugation is a simple procedure which can be of great utility in the measurement and investigation of free drug levels.
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