A vitreous drug finding may document detection without permitting a reliable estimate of peripheral blood concentration. Hubbard and colleagues demonstrated this distinction in 89 autopsy cases involving seven acidic drugs: detection rates and blood–vitreous relationships differed markedly by compound. The study offers evidence for selected alternative-matrix testing, not a general conversion formula.
The study was selected casework, not a prevalence survey
Over two years, the investigators included cases already tested for these drugs in peripheral blood that also had sufficient vitreous. Blood was collected at autopsy from the visually identified common iliac vein, up to 20 mL, into glass containing 100 mg sodium fluoride and 20 mg potassium oxalate. Vitreous from both eyes was stored in glass without preservative. Both were kept at 4°C until analysis. Individual death-to-collection intervals were not reported, and antemortem concentrations were unavailable.
The seven compounds were acetaminophen, ibuprofen, naproxen, salicylic acid, carbamazepine, phenobarbital and phenytoin. The total of 89 decedents should not be mistaken for 89 observations of each drug.
Detection depended on the analyte
| Drug | Blood findings | Vitreous findings |
|---|---|---|
| Acetaminophen | 50 | 50 |
| Naproxen | 17 | 17 |
| Ibuprofen | 9 | 4 |
| Carbamazepine | 8 | 8 |
| Phenobarbital | 4 | 4 |
| Phenytoin | 3 | 2 |
| Salicylic acid | 4 | 4 |
A negative vitreous result therefore did not exclude blood detection of ibuprofen or phenytoin. Conversely, matching detection counts for another drug do not prove that the concentrations were interchangeable.
Correlation and equality answer different questions
Vitreous concentrations were generally lower. Differences reached statistical significance for acetaminophen, carbamazepine, naproxen and ibuprofen, but not for salicylic acid, phenobarbital or phenytoin. The latter groups each involved four or fewer blood-positive cases; nonsignificance is not evidence that the matrices are equivalent.
Carbamazepine showed an R² of 0.995, and phenobarbital and phenytoin each exceeded 0.95. Those results suggest a relationship worth investigating, but the tiny phenobarbital and phenytoin subsets limit predictive confidence. Among the nonprescription analgesics, acetaminophen had an R² of 0.718 and naproxen 0.322. A high correlation, moreover, can coexist with systematically lower vitreous concentrations.
One naproxen pair, 8.1 mg/L in blood and 25 mg/L in vitreous, was treated as an outlier and excluded from subsequent analyses. That exclusion is material to interpreting the reported relationships; it should not disappear when the study is summarized.
The analytical limits are part of the finding
Analysis used HPLC with diode-array detection, not mass spectrometry. Identification incorporated relative retention time within ±0.05 and an ultraviolet library match of at least 95%. Calibration used at least three nonzero levels, R² ≥ 0.98, and calibrator/control acceptance within 20% of target.
The methods list lower quantification limits of 2 mg/L for acetaminophen and ibuprofen, 2.5 for carbamazepine, 5 for naproxen, phenobarbital and phenytoin, and 20 for salicylic acid. Yet some tabulated concentrations fall below those listed limits. The authors also acknowledge incomplete vitreous validation, including uninvestigated extraction efficiency and appropriate detection limits. Low tabulated numbers therefore should not be presented as fully validated quantitative performance below the stated limits.
What a case interpretation can support
The study supports drug-specific evaluation of vitreous as an alternative specimen. It does not establish antemortem concentrations, prove absence of redistribution, or identify impairment or cause of death from a vitreous number alone. The laboratory’s actual method and validation must support the proposed inference. Our guide to screening and confirmation records explains why analytical stage and reporting scope matter.
Primary source
Hubbard JA, Navarrete AL, Fitzgerald RL, McIntyre IM. Acidic Drug Concentrations in Postmortem Vitreous Humor and Peripheral Blood. Journal of Analytical Toxicology. Published online 2019;45(1), 2021. doi:10.1093/jat/bkz076.
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