  Citation: Zilg, B.; Alkass, K.; Kronstrand, R.; Berg, S.; Druid, H. A Rapid Method for Postmortem Vitreous Chemistry—Deadside Analysis. Biomolecules 2022, 12, 32. https://doi.org/10.3390/ biom12010032 Academic Editors: Nina Heldring and Brita Zilg Received: 1 December 2021 Accepted: 23 December 2021 Published: 27 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biomolecules Article A Rapid Method for Postmortem Vitreous Chemistry—Deadside Analysis Brita Zilg 1 , Kanar Alkass 1 , Robert Kronstrand 2 , Sören Berg 3 and Henrik Druid 1, * 1 Forensic Research Laboratory, Department of Oncology-Pathology, Karolinska Institute, 171 77 Stockholm, Sweden; brita.zilg@ki.se (B.Z.); kanar.alkass@ki.se (K.A.) 2 Department of Forensic Genetics and Forensic Toxicology, National Board of Forensic Medicine, 587 58 Linkoping, Sweden; robert.kronstrand@rmv.se 3 Division of Clinical Chemistry and Pharmacology, Department of Biomedical and Clinical Science, Faculty of Medicine and Health Science, Linköping University, 581 85 Linkoping, Sweden; soren.berg@liu.se * Correspondence: henrik.druid@ki.se; Tel.: +46-70-602-7141 Abstract: Vitreous fluid is commonly collected for toxicological analysis during forensic postmortem investigations. Vitreous fluid is also often analyzed for potassium, sodium, chloride and glucose for estimation of time since death, and for the evaluation of electrolyte imbalances and hyperglycemia, respectively. Obtaining such results in the early phase of a death investigation is desirable both in regard to assisting the police and in the decision-making prior to the autopsy. We analyzed vitreous fluid with blood gas instruments to evaluate/examine the possible impact of different sampling and pre-analytical treatment. We found that samples from the right and left eye, the center of the eye as well as whole vitreous samples gave similar results. We also found imprecision to be very low and that centrifugation and dilution were not necessary when analyzing vitreous samples with blood gas instruments. Similar results were obtained when analyzing the same samples with a regular multi-analysis instrument, but we found that such instruments could require dilution of samples with high viscosity, and that such dilution might impact measurement accuracy. In conclusion, using a blood gas instrument, the analysis of postmortem vitreous fluid for electrolytes and glucose without sample pretreatment produces rapid and reliable results. Keywords: vitreous; postmortem; glucose; electrolytes; forensic medicine 1. Introduction Autopsy has long been considered the gold standard in reaching a diagnosis when a person has died of uncertain causes. While this is still true regarding a large number of illnesses that are visible macroscopically or microscopically, there are many serious medical conditions that may escape detection. The major drawback is that autopsy diagnostics are traditionally based on morphology. Although computer tomography and magnetic resonance imaging has been introduced in the routine casework at many forensic medicine facilities in the last few decades, these radiological methods can also only provide structural information. Forensic toxicology is the only exception from the tradition of morphological diagnostics, which allows for the detection and quantification of alcohol and drugs by means of chemical analysis. With the help of postmortem reference concentrations, the pathologist may be able to diagnose, or rule out, an intoxication as the cause of death [1]. Toxicology was most likely introduced because intoxication does not usually cause any visible morphological signs/traces at autopsy. Chemical analyses of postmortem samples today are not limited to toxicology; anal- ysis of endogenous biomolecules in postmortem samples can also be used to identify pathologies. For instance, an increase in glial fibrillary acidic protein or neurofilament light protein in cerebrospinal fluid or serum can indicate brain injury [2] and increased troponin T [3] may be used as an indicator of myocardial infarction. Negative results can be equally important in the evaluation of the possible causes of death in the early stages Biomolecules 2022, 12, 32. https://doi.org/10.3390/biom12010032 https://www.mdpi.com/journal/biomolecules