Morphine pharmacokinetics: GLC assay versus radioimmunoassay

The validity of a radioimmunoassay (RIA) for research on the pharmacokinetics of morphine has been questioned because of the possible measurement of cross-reactive metabolites. An RIA using antiserum derived from the 3-O-carboxymethylmorphine hapten was compared with a specific GLC assay in the measurement of plasma morphine concentrations in humans. The ratio of values for morphine concentrations measured using RIA and those measured using GLC was determined. The RIA values resulted in a 27% overestimation of this ratio. This overestimation did not significantly affect the values for terminal elimination half-life, volume of distribution at steady state, or total body clearance that were derived using results from each assay and model-independent pharmacokinetic techniques.