Vitreous humor can support postmortem drug screening when blood is unavailable, but a negative vitreous result does not reliably exclude drugs detected in blood. In a paired study of 51 autopsies, Metushi, Fitzgerald and McIntyre found fewer drug findings and fewer distinct compounds in vitreous than in peripheral whole blood. Their evidence supports an alternative qualitative specimen, with important limits on quantitative interpretation.
What specimens and method were actually compared?
The investigators collected approximately 20 mL of peripheral blood from the visually identified left common iliac vein during autopsy. Blood tubes contained 100 mg sodium fluoride and 20 mg potassium oxalate. Approximately 5 mL of vitreous was collected from both eyes into glass without preservative. Both matrices were stored at 4°C. The article does not provide a case-by-case death-to-collection interval, so the results cannot define performance across all postmortem intervals.
Samples underwent protein precipitation and solid-phase extraction followed by full-scan electron-impact GC–MS. Identification required relative retention time within ±0.08 of the specified criterion and at least a 70% mass-spectral library match. This was a broad screening procedure with quantitative or semiquantitative capability for selected compounds; it was not a universally validated confirmatory assay for every drug.
Fewer findings are an important specimen-selection result
The blood specimens yielded 209 aggregate findings involving 71 different compounds; vitreous yielded 169 findings involving 60 compounds. These are findings across 51 cases, not 209 or 169 independent people. They also are not universal sensitivity estimates: the study did not establish an independent exposure reference standard for every possible analyte.
Morphine appeared in blood in 10 cases and vitreous in five. Seven-aminoclonazepam appeared in two blood specimens and no vitreous specimens. By contrast, diphenhydramine appeared in 11 specimens of each matrix. Such differences show why an alternative specimen must be evaluated by analyte and method rather than accepted or rejected as a single category.
Concentrations cannot simply be transferred between matrices
Among drugs eligible for quantitative comparison, blood concentrations were significantly higher for trazodone and diazepam (P < 0.05). The reported blood and vitreous values were 1.42 ± 0.57 versus 0.15 ± 0.05 mg/L for trazodone, and 0.37 ± 0.06 versus 0.13 ± 0.01 mg/L for diazepam. Trends toward higher vitreous concentrations of oxycodone, hydrocodone and doxylamine were not statistically significant. They should not be converted into claims that those drugs consistently concentrate in the eye.
What the validation leaves unresolved
Water and phosphate-buffered saline were unsatisfactory substitutes for vitreous calibrators in preliminary recovery work. The investigators therefore used pooled drug-free human vitreous. Even so, extensive vitreous validation was not performed: extraction efficiency for every compound and limits of detection and quantification were not established. The method was not optimized for opiates, and gabapentin quality-control performance was inadequate for optimal quantitation.
These limitations matter most when a negative finding is offered as proof of no exposure, or a screening estimate is presented as a precise concentration. Neither inference follows automatically. Detection establishes an analytical finding in the collected specimen; this study did not validate a concentration threshold for impairment or establish that a detected drug caused death.
Applying the study to a laboratory report
Identify the exact specimen, collection site, storage conditions, analyte and reporting threshold. Determine whether the result is qualitative, semiquantitative or supported by a separate quantitative method. For the broader distinction between analytical stages, see our discussion of screening, confirmation and laboratory records; its methamphetamine example does not supply vitreous validation.
Primary source
Metushi IG, Fitzgerald RL, McIntyre IM. Assessment and Comparison of Vitreous Humor as an Alternative Matrix for Forensic Toxicology Screening by GC–MS. Journal of Analytical Toxicology. 2016;40:243–247. doi:10.1093/jat/bkw009.
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