A pressure ultrafiltration method for serum free drug determination. Comparison with equilibrium dialysis using...

The clinical methods most frequently used for serum free drug determinations are equilibrium dialysis and ultrafiltration. Both methods have usually been considered equal when comparisons have been made on normal sera. The purpose of the study was to develop a reliable and rapid method for serum free drug determinations especially for disease states where there is an increased free fraction. This modification of the pressure ultrafiltration method proved to meet these demands. The method was tested in vitro by adding radiolabelled phenytoin in different concentrations to normal and uraemic sera. The within-run and between-run variation coefficients were both under 5%. In comparison with equilibrium dialysis the values obtained by the ultrafiltration method were 0.65% lower (p less than 0.001, Student's paired t-test) in normal and 0.37% lower (non-significant) in uraemic sera. There are some theoretical grounds to presume that the results obtained by means of the pressure ultrafiltration method would better reflect the real values than those obtained by means of equilibrium dialysis.