Stability of PEth 16:0/18:1, 16:0/18:2, 16:0/20:4, 18:0/18:1, 18:0/18:2, and 18:1/18:1 in authentic whole blood samples (at room temperature).

Phosphatidylethanol (PEth) is a direct alcohol biomarker to monitor individuals' drinking behavior that has gained recognition in clinical and forensic settings. The increasing application of the marker makes investigation of the preanalytical handling necessary, and analyte stability deserves major attention. This study was conducted to investigate the change of six PEth homologues' concentration, stored in authentic samples of EDTA blood over a course of 30 days at room temperature (n = 62). The stability criterion of concentration being ±15% of the original concentration was fulfilled at mean for 10, 3, 2, 5, 2, and 7 days for PEth 16:0/18:1, 16:0/18:2, 16:0/20:4, 18:0/18:1, 18:0/18:2, and 18:1/18:1, respectively. Regarding all homologues, there were samples in which concentration had declined by >15% or by more than the critical difference on day 1. Overall, calculated concentration declines were very inhomogeneous, with inter-sample differences of 43%-73% after 30 days. PEth 16:0/18:2, 16:0/20:4, and 18:0/18:2 declined to a greater extent than PEth 16:0/18:1. Blood alcohol concentration was measured >0.1‰ in 25 samples. Three of the six samples that exceeded 115% of initial concentrations were positive for blood alcohol. The study results add to the previously reported information on PEth stability and firstly look at six homologues in comparison. Due to the high scatter of stability among the samples and the observed poor stabilities in some, it can be concluded that transportation and storage times, especially if cooling cannot be provided, must be kept short. If analyzing from dried blood, spotting should preferably be conducted at the site of sampling.