Comparative determination of flupentixol in plasma by gas chromatography and radioimmunoassay in schizophrenic patients.

A new method of measuring flupentixol in plasma was developed using gas chromatography (GC) with a nitrogen-phosphorus detector. The minimal quantifiable concentration was 0.5 ng/ml and the day-to-day coefficient of variation was 9.4% at 2 ng/ml. A comparison of flupentixol concentrations measured with radioimmunoassay (RIA) and GC was undertaken on 100 plasma samples obtained from 50 schizophrenic patients. The RIA method tended to overestimate flupentixol concentrations compared with GC. The reasons for this discrepancy might be the presence in plasma of the inactive trans(E)-flupentixol isomer and metabolites of flupentixol, which would cross-react with the antibody. The results of this study show that GC can be used for therapeutic monitoring of flupentixol.