Quantitation of phosphatidylethanol in dried blood after volumetric absorptive microsampling

BACKGROUND

Stimulated by the increased recognition of phosphatidylethanol (PEth) as sensitive direct marker of alcohol intake, the Ghent University's Laboratory of Toxicology and the National Institute of Criminalistics and Criminology combined their efforts to develop a quantitative method. To facilitate implementation the focus was on the use of a sampling technique which allows quick and easy blood collection, without the need of dedicated personnel at any place/any time. In the meantime the cooperation of the two labs should also allow to initiate a Belgian network of laboratories capable of quantifying PEth.

METHODS

Dried blood microsamples were collected via volumetric absorptive microsampling (VAMS). PEth 16:0/18:1 was quantified after liquid-liquid extraction using two independent isotope dilution - liquid chromatography - tandem mass spectrometry methods. A systematic review of the entire process at both sites was performed before the final method comparison using samples from 59 routine toxicology cases collected within a one-year time interval.

RESULTS

Initial differences between both laboratories were solved by focusing on important methodological aspects: (i) trueness verification of the calibration protocol focusing on the primary material, preparation of the stock solutions and adequate equilibration of calibrators and QCs, and (ii) verification of comparability of results obtained with different m/z transitions. Several of these aspects could only be verified by critically assessing spiked and native samples. After a final validation good average comparability of the two methods was observed. The average bias was -0.4%, with 85% of the differences within 20%. Moreover, the methods proved to be reproducible and robust within a one-year time interval.

CONCLUSION

This study is the first to develop a quantitative volumetric absorptive microsampling based method for PEth measurements, in addition it is the first to perform a systematic comparison of PEth measurements between two laboratories. From the discussion on the encountered pitfalls it is clear that also on a global scale, more efforts are needed to improve interlaboratory agreement.

Evaluation of N-acetyltaurine as an ethanol marker in human blood

To investigate the potential of N-acetyltaurine (NAcT) in blood as a biomarker for alcohol uptake, a previously published LC-MS/MS method for urine was modified to simultaneously detect NAcT and ethyl glucuronide (EtG). The method was applied in a drinking study and by analyzing 147 forensic case samples. In the drinking study, contrary to EtG, NAcT proved to be an endogenous substance, which was present at 22 ± 7 ng/mL (13-31 ng/mL) in the blood after 2 weeks of abstinence. A moderate increase in NAcT to 40 ± 10 ng/mL (27-57 ng/mL) was observed after drinking. Within 24 h, the NAcT concentrations declined to starting concentrations in seven out of eight subjects. Peak EtG concentrations (c¯_max) of 445 ± 101 ng/mL (278-662 ng/mL) were reached. While EtG in blood can be used to detect alcohol consumption even if ethanol is already eliminated, some of the maximum NAcT concentrations after a single ethanol dose were in the range of endogenous levels detected prior to the start of drinking in other subjects. In the 147 blood samples, the following concentrations were found: blood alcohol concentration (BAC): 1.22 ± 0.95 g/kg (0-3.46 g/kg); NAcT: 37.8 ± 18.4 ng/mL (12.1-109 ng/mL); EtG: 1149 ± 1121 ng/mL (0-5950 ng/mL). ROC curve analysis for BAC thresholds at 0.8 and 1.6 g/kg were performed for EtG and NAcT. Due to the presence of endogenous NAcT levels resulting in a lower sensitivity and selectivity when compared to EtG, and due to a minor increase in concentration after alcohol uptake, the usefulness of NAcT as an alcohol biomarker in blood is very limited.