Fentanyl: toxic or therapeutic? Postmortem and antemortem blood concentrations after transdermal fentanyl application

In forensic toxicology, several fatal intoxications with fentanyl have occurred in the recent past, but there are rare discussions in the literature of postmortem fentanyl blood concentrations subsequent to lethal and non lethal applications. To study this problem, we analyzed postmortem blood concentrations (vena femoralis) of 118 cases with therapeutic use of fentanyl and compared them with serum levels of 27 living persons after therapeutic administration of fentanyl patches (Durogesic). Basically, blood concentrations in postmortem specimens cannot be directly compared with in vivo serum levels: in our study, we observed that postmortem fentanyl blood concentrations were on average up to nine times higher than in vivo serum levels at the same dose. These differences could be explained by postmortem redistribution, but they were higher than expected on the basis of the physical and chemical properties of fentanyl alone. The special pharmacokinetics of the drug after long term transdermal application seem to play an important role in this phenomenon. In addition, there was no clear correlation between transdermal fentanyl dose and blood or serum concentrations, either antemortem or postmortem. Our study provides extensive data for postmortem peripheral blood concentrations after therapeutic non-fatal fentanyl patch application and demonstrates once more that in forensic toxicology, blood concentrations must be holistically interpreted with respect to all aspects of a case.