Monitoring of plasma methadone: intercorrelation between immunoassay and gas chromatography-mass spectrometry

Determination of plasma methadone is essential in connection with dose adjustments for patients participating in methadone maintenance programs. We successfully adapted the existing fluorescence polarization immunoassay (FPIA) kit intended for urinary methadone to plasma assays. A concentration interval of 50-900 ng/ml could be covered. The coefficient of variation was less than 7%, and the limit of detection below 50 ng/ml. The intercorrelation between the immunoassay and a specific gas chromatographic-mass spectrometric (GC-MS) method was studied in samples from 19 heroin addicts in methadone maintenance treatment. A total number of 97 plasma samples with a concentration range of 31-842 ng/ml were used. The slope and intercept of the regression line (CFPIA = 0.93 X CGC-MS + 15) was in good agreement with the theoretical relation (CFPIA = CGC-MS), with a coefficient of correlation of 0.978. The mean ratio, in quantitative result, between the techniques (CFPIA/CGC-MS) was 1.03 +/- 0.01 (SEM). We conclude that the immunoassay proposed in this study can be safely used in patients participating in methadone maintenance programs.