Postmortem Redistribution: Why C/P Ratios Are Not Correction Factors

Quick answer: A central-to-peripheral (C/P) blood concentration ratio can help describe postmortem redistribution, but it should not be used as a universal mathematical correction factor to reconstruct an antemortem drug concentration. Postmortem redistribution is drug-specific, case-specific, site-specific, time-dependent, and influenced by sampling technique, decomposition, chronic use, tissue reservoirs, and postmortem interval.

What is postmortem redistribution?

Postmortem redistribution (PMR) is the change in drug concentration that can occur after death as drugs move between tissues and blood. Central blood, particularly cardiac blood, may be more affected because of nearby high-concentration organs and tissues such as the liver, lungs, and myocardium. Peripheral venous blood is generally preferred for quantitative postmortem interpretation, but even peripheral blood is not immune from postmortem change.

This is one reason a postmortem concentration should not be treated as though it were simply a preserved antemortem blood sample. The measured value reflects both the decedent’s exposure and the biology and sampling conditions that occurred after death.

What does a C/P ratio actually tell you?

A C/P ratio compares the concentration measured in a central blood specimen with the concentration measured in a peripheral blood specimen. A ratio above 1 may be consistent with a higher central concentration, which is often seen with drugs that redistribute after death. But the ratio is descriptive, not a universal conversion equation.

The forensic literature demonstrates substantial interindividual variability. In a 2024 study of chronic methadone users, reported central-to-peripheral behavior was atypical: selected cases had peripheral concentrations higher than cardiac concentrations, producing C/P ratios below 1. The authors emphasized that sampling more than one blood source can materially affect interpretation. See Garneau et al., Journal of Analytical Toxicology (2024).

Why a published average ratio is not a correction factor

A common interpretive error is to take a published mean or median C/P ratio for a drug and divide a measured postmortem concentration by that number to estimate the concentration before death. That approach assumes the case behaves like the study average and that the measured sites, postmortem interval, decomposition state, dose history, tissue burden, and sampling technique are comparable. Those assumptions are usually unproven.

Older paired antemortem/postmortem work has shown that attempting to reconstruct antemortem concentrations from postmortem samples can produce large errors and that blood collection site materially affects the measured concentration. See Cook, Braithwaite & Hale, Journal of Clinical Pathology.

Sampling site matters

Not all peripheral specimens are equivalent. Femoral, iliac, jugular, arterial, and venous blood can differ. A study comparing multiple postmortem sampling sites found systematic differences between central and peripheral blood and between arterial and venous specimens, with larger differences in cases with longer postmortem intervals. The authors recommended peripheral venous blood when available. See postmortem blood sampling comparison data.

The collection record therefore matters. A toxicology report that simply says “blood” may omit information essential to interpretation. The expert should determine the anatomical source, whether the vessel was isolated before collection, whether the specimen was pooled, whether resuscitation or trauma altered the compartment, and whether more than one blood source was tested.

Drug properties help predict risk, but do not solve the case

Drugs with a large apparent volume of distribution, high tissue binding, lipophilicity, or substantial tissue reservoirs may be more prone to PMR. But those physicochemical properties do not produce a single reliable correction coefficient. Different drugs can redistribute differently, and the same drug can behave differently across cases.

When blood is unavailable or compromised, alternative tissues may provide additional evidence, but they require matrix-specific interpretation. See Brain Tissue in Postmortem Toxicology: Brain–Blood Ratios and Interpretation for the current evidence on brain concentrations and why brain–blood ratios should not be treated as universal conversion factors.

Controlled and case-based studies also show that postmortem interval and storage conditions can influence concentration patterns. THC research, for example, has demonstrated wide central-to-peripheral ratios across postmortem conditions rather than one stable relationship. See time- and temperature-dependent postmortem THC data.

Chronic use can complicate redistribution

Repeated exposure may change tissue burdens and create behavior that differs from an acute exposure model. The methadone case series noted above is important because it demonstrates that even the expected direction of central-versus-peripheral difference may not hold in every case. That makes a population-average C/P value particularly risky when used as an individual correction factor.

What evidence should be reviewed before interpreting PMR?

• Exact blood collection site and collection technique.
• Postmortem interval and body storage conditions.
• Decomposition status.
• Antemortem hospital specimens, if any.
• Central and peripheral blood results when both are available.
• Vitreous fluid, urine, liver, brain, or other matrices where scientifically relevant.
• Medication history and evidence of chronic versus acute exposure.
• Resuscitation, transfusion, IV fluids, trauma, and survival interval.
• Analytical method, calibration range, dilution history, uncertainty, and validation records.

Can vitreous fluid help?

Vitreous fluid is anatomically more isolated than blood and can be less susceptible to some postmortem processes. It is not a universal substitute for blood, and drug partitioning into vitreous is compound-specific, but paired-matrix analysis can provide useful context. A 2025 Journal of Analytical Toxicology study comparing peripheral blood and vitreous across multiple drugs found vitreous useful for several analytes while also demonstrating matrix-dependent detection and concentration behavior. See the 2025 paired blood-vitreous study.

Forensic interpretation: the defensible conclusion

The scientifically defensible use of PMR data is generally qualitative or contextual: identify whether the drug is known to redistribute, evaluate whether the observed specimen pattern is consistent with that phenomenon, and explain the resulting uncertainty. A C/P ratio may inform that analysis. It ordinarily should not be presented as a validated universal equation that converts a postmortem concentration into a precise antemortem value.

For a related discussion of why therapeutic, toxic, and fatal concentration tables overlap, see Postmortem Drug Concentrations: Why Reference Ranges Overlap. For preanalytical issues that can further affect specimen interpretation, see Specimen Storage and Stability in Forensic Toxicology.

Key takeaway

C/P ratios are evidence about redistribution, not automatic correction factors. The strongest interpretation integrates specimen site, postmortem interval, matrix, case history, analytical method, and drug-specific literature rather than forcing an individual case into a population-average ratio.

Technical educational material only. Interpretation in a specific case requires review of the complete toxicology and medicolegal record.