FORENSIC TOXICOLOGY.NETOkorie Okorocha, J.D., M.S., M.S.Forensic Toxicology Expert Witness

EVIDENCE LIBRARY

Brain Tissue in Postmortem Toxicology: Brain–Blood Ratios and Interpretation

By forensic toxicology working group members

Brain tissue can be valuable when postmortem blood is unavailable or compromised, but brain–blood ratios and tissue concentrations require matrix-specific validation and cautious interpretation.

Direct answer: Brain tissue can be a valuable alternative specimen in postmortem toxicology when blood is unavailable, degraded, redistributed, or otherwise difficult to interpret. But a brain concentration is not a simple substitute for a blood concentration, and a brain–blood ratio should not be treated as a universal conversion factor. Interpretation must account for the specific drug, sampling location, analytical method, tissue homogenization, matrix effects, postmortem interval, tolerance, co-intoxicants, and the investigative context.

A major 2026 study adds useful reference data to this area. Skov and colleagues reported brain concentrations and brain–blood ratios for 122 compounds across 2,264 postmortem cases collected over 11 years. The study found substantial overlap between cases categorized as having toxic versus therapeutic blood concentrations, both in blood and brain, and reported especially high brain–blood ratios for many centrally acting drugs such as antidepressants and sedative-hypnotics. That makes the data useful as a reference framework, but it does not convert brain tissue into a stand-alone cause-of-death test.

Why brain tissue may be useful after death

Postmortem blood can be limited by exsanguination, decomposition, severe trauma, skeletonization, redistribution, contamination, or an uncertain sampling source. Brain tissue is comparatively protected in many of these circumstances and can preserve evidence of exposure to centrally acting substances. Professional postmortem-toxicology guidance recognizes brain, liver, spleen, and other tissues as potentially useful alternative matrices when conventional fluids are unavailable.

The principal value of brain tissue is therefore often evidentiary and interpretive: it can help establish that a drug reached a pharmacologically relevant organ, support or challenge findings in another specimen, or provide an analyzable matrix when blood cannot be obtained. It should not be assumed that the concentration has the same meaning as the concentration measured in peripheral blood.

What the 2026 brain–blood study adds

The September 22, 2026 Journal of Analytical Toxicology study, “Thinking beyond blood: Brain concentrations and brain-blood ratios in postmortem toxicology”, is important because large reference datasets for brain tissue have historically been sparse. The investigators evaluated 2,264 cases and reported concentrations and brain–blood ratios for 122 drugs, pharmaceuticals, metabolites, and drugs of abuse.

Three findings are especially important for forensic interpretation:

  • Therapeutic and toxic distributions overlap. A concentration falling within a reported “toxic” or “therapeutic” range does not by itself establish or exclude toxicity.
  • Brain–blood ratios vary by compound. Centrally acting drugs, including many antidepressants and sedative-hypnotics, may show comparatively high brain–blood ratios.
  • Reference values are context, not conversion equations. Population data may help frame an individual result, but they do not justify mechanically back-calculating an unknown blood concentration from a measured brain concentration.

Why a brain–blood ratio is not a universal correction factor

A ratio describes the relationship between two measured concentrations under the conditions of a particular case or dataset. It does not prove that the same relationship existed earlier in life, will apply across all postmortem intervals, or will remain constant across sampling regions, decomposition states, analytical procedures, or individuals.

Drug distribution into brain tissue depends on properties such as lipophilicity, protein binding, pKa, transporter activity, tissue affinity, and pharmacokinetics. After death, additional processes can alter measured concentrations. The same caution that applies to central-to-peripheral postmortem blood ratios also applies here: a population ratio is descriptive evidence, not a validated case-specific correction equation.

Sampling location inside the brain matters

“Brain” is not a chemically uniform specimen. Gray matter, white matter, cerebellum, cortex, brainstem, and other regions differ in lipid content, blood supply, and cellular composition. A laboratory report that simply says “brain” may therefore omit information important to interpretation.

Case review should identify, when available, the anatomical sampling site, whether the specimen was a discrete region or pooled tissue, the degree of decomposition, and whether the tissue was homogenized before aliquoting. These issues are related to the broader problem of specimen homogeneity and aliquot representativeness.

Analytical validation must cover the tissue matrix

A method validated for whole blood is not automatically validated for brain homogenate. Brain tissue introduces different extraction behavior, endogenous interferences, lipid content, ion suppression or enhancement, recovery, and homogenization requirements. A laboratory using brain for quantitative reporting should have evidence that its analytical procedure is fit for that matrix or has otherwise demonstrated acceptable performance for the intended use.

Relevant questions include:

  • Was brain tissue included in method validation or matrix-equivalence studies?
  • What matrix was used for calibrators and controls?
  • How was the brain specimen homogenized and at what tissue-to-diluent ratio?
  • Were matrix effects, recovery, precision, bias, carryover, and stability evaluated?
  • Were results reported per gram of tissue, per milliliter of homogenate, or using another convention?
  • Were dilution factors and wet-weight calculations documented?

For additional background, see Matrix-Matched Calibration in Forensic Toxicology and How Matrix Effects Are Evaluated in LC-MS/MS Forensic Toxicology.

Brain concentrations do not establish impairment or cause of death by themselves

A measured tissue concentration is one piece of a medicolegal reconstruction. Cause-of-death interpretation should integrate the toxicology result with autopsy findings, scene evidence, medical history, prescription history, tolerance, route of administration, survival interval, co-detected substances, and the known pharmacology of the drug.

This is consistent with professional postmortem-toxicology best practices, which emphasize integrating analytical findings with investigative and autopsy information rather than treating concentration tables as self-executing conclusions. The 2026 brain study itself found substantial therapeutic–toxic overlap, reinforcing the need for case-specific interpretation.

Case-review checklist for brain-tissue toxicology

Question Why it matters
What exact brain region was sampled? Different regions may differ in composition and drug distribution.
Was the tissue homogenized, and how? Homogenization affects representativeness and reported concentration.
Was the method validated for brain tissue? Blood validation alone does not establish quantitative performance in brain matrix.
What units and dilution calculations were used? Tissue results may be reported using conventions that are not directly comparable with blood.
Was paired peripheral blood available? Paired specimens can provide additional context but do not create a universal conversion factor.
What was the postmortem interval and decomposition state? Postmortem change may affect both specimen quality and interpretation.
Were other drugs or alcohol present? Polydrug effects can matter more than any single concentration.
Is the opinion based on a reference distribution or a validated causal threshold? Reference ranges and ratios should not be overstated as deterministic cutoffs.

How to use published brain–blood reference data responsibly

Large reference datasets are valuable because they show the range and variability encountered in real casework. They can help identify whether an observation is unusual, provide context where blood is unavailable, and support hypothesis generation. The defensible use is comparative and contextual. The risky use is deterministic: converting a ratio into an assumed antemortem blood concentration, assigning a cause of death from a tissue value alone, or treating overlapping population distributions as bright-line thresholds.

Authoritative sources

Technical review: Forensic Toxicology Working Group Members. This reference is educational and addresses scientific interpretation, not the facts or ultimate legal conclusion in any specific case.