Rapid measurement of plasma methadone in a clinical setting using florescence polarization immunoassay

Currently clinicians using methadone for treatment of opioid dependency or organic pain lack a simple, real-time, analytical method useful for adjusting methadone dose. Therefore, we evaluated the feasibility of measuring plasma methadone concentrations using florescence polarization immunoassay (FPIA) in an outpatient clinical setting. Using this technology, patient plasma methadone concentrations can be determined in approximately 20 minutes. The FPIA calibration curve was found to be linear over the methadone concentration range of 0 to 1600 ng/ml. Day-to-day run coefficient of variation was 5-10%, the within-run coefficient of variation was 1.3-6.3%; the limit of quantification for the assay was 25 ng/ml. Calibration plots of HPLC and GCMS (mass fragment 72) plasma methadone control samples versus FPIA were also linear. A plot of HPLC plasma methadone versus FPIA for patient samples was identical to results for control samples (concentration range 0-1200 ng/ml). No significant amounts of the EDDP methadone metabolite were found in plasma. Based upon these findings, FPIA is clinically useful for monitoring plasma methadone concentration in outpatient settings.