Evaluation of various internal standards for quantification of dextromethorphan and diphenhydramine in plasma: a fatal overdose case of a mid-teenager caused by personally imported and over-the-counter medicines.

Purpose: Over-the-counter medicines are commonly used for recreational and suicidal overdoses, a global problem. Some of these are easily obtained via the Internet. In cases of intoxication, drug quantification is necessary to estimate the cause of death. Stable isotope compounds are recommended as internal standards (IS) for analyzing drugs; however, it is difficult for individual laboratories to obtain isotopes for all analytes due to cost and availability. Therefore, alternative IS selection is important for practicality. Here, we quantified diphenhydramine and dextromethorphan concentrations in plasma from several collection sites in a fatal intoxication case, and assessed various IS performance based on structural similarities and retention time.

Methods: A mid-teenager died from intoxication of personally imported dextromethorphan and Over-the-counter diphenhydramine. To quantify these drugs, we selected morphine-d₃, dihydrocodeine, diphenhydramine-d₃, mianserin-d₃, and diazepam-d₅ as alternative IS and evaluated. After selecting the most suitable IS, we quantified dextromethorphan and diphenhydramine concentrations in twelve plasma samples from the victim by liquid chromatography-tandem mass spectrometry.

Results: Recovery rates were 80.7-105.5%, except for morphine-d₃ (47.8%) and dihydrocodeine (64.8%). Matrix effects were 75.7-103.2%. The intra-day accuracies and precisions were 86.4-119.5% and 0.27-12.2%, respectively. The inter-day accuracies were 81.2-119.8%, and the precisions were 0.80-9.44%. The validation study showed that diphenhydramine-d₃ was the most suitable IS. Finally, plasma concentrations of dextromethorphan and diphenhydramine were 3.74-10.3 µg/mL and 15.6-52.9 µg/mL, respectively.

Conclusions: The concentrations of both drugs in plasma samples were estimated to cause death. When using an alternative IS, a validation study is needed to select the optimal IS.