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 measurement of morphine and its active metabolite, morphine-6-glucuronide (M6G), in plasma is described. Plasma samples were assayed following appropriate dilution, using a morphine specific antiserum and the results subtracted from those obtained with an antiserum which cross-reacts with both morphine and M6G. The sensitivity of measurement for morphine and M6G was 0.88 and 0.27 nmol l-1, respectively and inter-assay variation ranged from 3.4 to 11.0%. Recovery of morphine and M6G was quantitative over a range of concentrations (1-5000 nmol l-1). The presence of either M6G or morphine-3-glucuronide (M3G) did not affect the recovery of morphine. M6G was quantitatively recovered in the presence of morphine but high concentrations (>1:20) of M3G caused some overestimation of M6G. Results obtained by differential RIA for both morphine and M6G correlated well with the results of HPLC analysis. The assay has been applied to the measurement of M6G in plasma following its administration to human volunteers.