EVALUATION OF A DIFFERENTIAL RADIOIMMUNOASSAY TECHNIQUE FOR THE DETERMINATION OF MORPHINE AND MORPHINE-6-GLUCURONIDE IN HUMAN PLASMA

A modified differential radioimmunoassay (RIA) technique for the measu rement of morphine and its active metabolite, morphine-6-glucuronide ( M6G), in plasma is described. Plasma samples were assayed following ap propriate dilution, using a morphine specific antiserum and the result s subtracted from those obtained with an antiserum which cross-reacts with both morphine and M6G. The sensitivity of measurement for morphin e and M6G was 0.88 and 0.27 nmol l-1, respectively and inter-assay var iation ranged from 3.4 to 11.0%. Recovery of morphine and M6G was quan titative over a range of concentrations (1-5000 nmol l-1). The presenc e of either M6G or morphine-3-glucuronide (M3G) did not affect the rec overy of morphine. M6G was quantitatively recovered in the presence of morphine but high concentrations (>1:20) of M3G caused some overestim ation of M6G. Results obtained by differential RIA for both morphine a nd M6G correlated well with the results of HPLC analysis. The assay ha s been applied to the measurement of M6G in plasma following its admin istration to human volunteers.