[Determination of plasma levels of phenytoin. Evaluation of 2 techniques]

We compared 2 techniques for determination of plasma levels of phenytoin, liquid-gas chromatography (LGC) and homogeneous enzymatic immunoassay (EI) in samples from 72 patients receiving monotherapy with this drug. Determinations were also made in samples of normal plasma with known concentrations of phenytoin. Good correlation of both techniques was observed in patient samples containing less than 15 micrograms/ml of the drug but not with higher concentrations. LGC was accurate in determination of known concentrations of phenytoin, whereas immunoassay showed an exponential deviation with overestimation of phenytoin levels. Thus LGC is a better technique for determination of phenytoin levels; however EI may be used in most clinical situations when the concentration of phenytoin lies under 20 micrograms/dl.