A satisfactory internal-standard signal does not guarantee accurate drug quantification in lipid-rich postmortem blood. A 2023 experiment found substantial losses of certain benzodiazepine metabolites and sedative drugs even when internal-standard responses were comparatively preserved. The investigators found evidence pointing primarily to extraction-related losses rather than ion suppression in the compounds specifically examined.
Why postmortem lipemia deserved investigation
Elenstål and colleagues reviewed approximately 28,000 autopsy cases analyzed in Sweden during 2017–2021; 1,070 were classified visually as hyperlipidemic. Severe putrefaction was recorded in 32% of the lipemic cases versus 5% overall. No putrefaction was recorded in 9% versus 61%, respectively.
This association did not prove that decomposition caused every lipid-rich specimen or that the person had high blood lipids during life. The classification depended on visible layering, turbidity or a milky appearance. The experimental work addressed the effect of that matrix on testing, not diagnosis of a living lipid disorder.
What was experimentally added?
The investigators spiked citrated bovine whole blood with 19 benzodiazepine-related, sedative and antianxiety analytes at 35, 84 or 140 ng/g, depending on the compound’s calibration range. In duplicate preparations, they added Intralipid at 0, 2.4, 5.6, 12, 24 and 64 mg/g.
A second additive pooled upper lipid layers from seven postmortem cases with negative toxicological screening. Those pooled lipids were added at 0, 8 and 52 mg/g, also in duplicate. These were matrix experiments, not measurements of drug concentrations in 19 independent human fatalities. The report did not assign a uniform collection site or postmortem interval to the seven source cases.
Samples equilibrated for five to ten minutes, then remained at 8°C for at least 24 hours before analysis. Internal standards were added during analytical preparation, after storage, followed by ethyl-acetate extraction and UHPLC–mass spectrometry.
Analyte and internal standard did not always behave alike
At 5.6 mg/g Intralipid, a level the investigators could not identify visually as hyperlipidemic, recoveries were below 50% for 7-aminonitrazepam, 7-aminoclonazepam, 7-aminoflunitrazepam, propiomazine and dihydropropiomazine. By contrast, internal-standard responses were 79–101% of the reference responses at that lipid level.
Several parent benzodiazepines retained responses above 75%, while zopiclone, zolpidem and hydroxyzine fell between 50% and 75%. Lipemia did not produce one uniform loss across all related drugs. For most analytes, standards compensated more successfully; the important limitation concerned the compounds and conditions for which they did not.
The study calculated its recovery measure by comparing absolute peak area in lipid-containing samples with a concurrent reference without added lipid. These relative responses are not a direct measurement of biological drug elimination or postmortem redistribution within the body.
Why ion suppression was not the whole explanation
Post-column infusion experiments examined alprazolam, 7-aminoclonazepam and propiomazine. No apparent ion suppression was observed for those three compounds under the tested conditions, despite major losses for two of them in the preparation experiment.
The authors proposed pre-ionization effects, including partitioning into lipid particles, incomplete homogenization or insufficient equilibration of the standards. Those are plausible explanations, not experimentally established mechanisms for every analyte. The result should not be generalized to mean that ion suppression never occurs in postmortem blood.
A surrogate revealed the problem without exactly reproducing authentic lipids
Both additives produced the same broad pattern, but Intralipid generally caused larger losses. The discussion gives approximate propiomazine recoveries of 2% with Intralipid versus 20% with pooled postmortem lipids. Intralipid was useful as a controlled challenge matrix, while not being numerically interchangeable with every authentic sample.
These findings support analyte-specific investigation of lipemia and internal-standard performance. They do not justify multiplying a case result by a universal correction, declaring every low result erroneous or inferring impairment or cause of death from the matrix appearance. For a related validation issue, see why authentic specimens can reveal assay interference.
Primary source
Elenstål E, Green H, Kronstrand R, Elmsjö A. Intralipid as a matrix additive for evaluating hyperlipidemic postmortem blood. Journal of Analytical Toxicology. 2023;47:529–534. doi:10.1093/jat/bkad025.
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