The influence of blood sample preparation on measured levels of morphine and its major metabolites.

The effect of six different methods of blood sample preparation on concentrations of morphine (M), morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) have been investigated using a specific high performance liquid chromatography assay. No difference between glass or plastic tubes was seen in concentrations of M, M3G, or M6G; and no difference between plasma and serum in M and M6G concentrations. M3G concentrations, however, were significantly lower in plasma compared with serum. Heparin had no effect on M, M6G or M3G, whereas citrate in a glass tube produced consistently lower concentrations of M, M3G and M6G. Standardization of sample collection using plasma samples in plastic heparin tubes is recommended for future studies.