The influence of preanalytical factors on concentrations of morphine and metabolites in patients receiving morphine.

To examine the importance of preanalytical factors on the sample concentration of morphine, morphine-3-glucuronide and morphine-6-glucuronide, blood samples were drawn in sets of four from 21 patients who were given morphine due to chronic pain or as premedication prior to surgery. Three different sampling tubes, different combinations of incubation temperature and time, and different temperatures during centrifugation were used and compared to reference standard treatment of blood samples using analysis of variance. Blood samples taken in EDTA tubes produced significantly higher (4.8%) concentrations of morphine compared with the reference levels obtained in heparin glass tubes. Incubation of blood samples at body temperature resulted in significantly higher (4.4%) morphine levels compared to reference levels. None of the other conditions studied influenced the sample concentrations of morphine significantly. The levels of the metabolites were not significantly affected under any of the tested conditions. Even if the preanalytical factors investigated in this study had little influence on the results, a standardised procedure is recommended for handling of blood samples for analysis of morphine and its metabolites.