EVIDENCE LIBRARY
A THC Concentration Is Not Complete Until the Specimen Matrix Is Identified
A THC concentration has no complete meaning without the specimen matrix. Plasma, whole blood, and postmortem blood fractions are not interchangeable, and a conversion factor derived from living subjects should not be carried into postmortem interpretation.
Ten nanograms per milliliter in plasma is not automatically the same evidence as ten nanograms per milliliter in whole blood. The number may use the same units while describing a different specimen compartment.
That distinction matters in cannabinoid interpretation because THC and its metabolites distribute differently between plasma and blood cells. It matters even more after death, when hemolysis, decomposition, protein changes, and postmortem redistribution can alter the relationship between fractions.
A 2001 Forensic Science International study by Giroud and colleagues examined these relationships in living cannabis users and postmortem cases. The paper supplies useful data, but it also shows why a specimen-conversion factor must not be treated as a universal constant.
What plasma and whole blood contain
Whole blood contains plasma plus red cells, white cells, and platelets. Plasma is the liquid fraction collected when clotting is prevented and cells are separated. Serum is the liquid remaining after clot formation in antemortem clinical collection.
Postmortem blood does not necessarily separate into a clean clinical serum. Giroud and colleagues placed the word "serum" in quotation marks when referring to the clear supernatant recovered after centrifuging autopsy blood. That terminology signals a scientific limitation: this material is not automatically equivalent to serum collected from a living patient.
THC binds extensively to plasma proteins and distributes unevenly between cellular and liquid fractions. A concentration therefore changes when the denominator changes from whole blood to plasma or a postmortem supernatant.
See Whole Blood and Plasma in Toxicology and Biological Matrices in Toxicology.
What the study measured in living subjects
The investigators obtained paired whole-blood and plasma measurements from eight regular cannabis users. Sixteen paired samples were included. They measured delta-9-THC, 11-hydroxy-THC, and the inactive carboxy metabolite THCCOOH.
Within that small living-subject dataset, plasma concentrations were higher than whole-blood concentrations. The authors found sufficiently similar average distribution ratios to suggest that a factor of approximately 1.6 could be used, with caution, to estimate plasma concentrations from whole blood.
The phrase "with caution" controls the interpretation. Eight participants do not define every person's partitioning. A mean ratio summarizes a group of paired observations. It does not prove the exact ratio in a specimen for which only one matrix was tested.
Preanalytical conditions can also matter. Hematocrit, storage, tube surfaces, separation technique, and the timing of processing may affect the measured relationship. A conversion adds assumptions to an already measured number.
Why the postmortem findings were different
The researchers also examined paired whole blood and centrifuged postmortem supernatant in six cases. The postmortem ratios were higher and more variable than those observed in living subjects. Across the three cannabinoids, the average postmortem supernatant-to-whole-blood ratio was approximately 2.4, with individual measurements dispersed across a wider range.
The authors discussed several possible mechanisms, including hemolysis, protein aggregation and degradation, and redistribution after death. Their conclusion was direct: the living-subject conversion should not be used to estimate the timing of cannabis exposure from postmortem cannabinoid concentrations.
That does not mean postmortem cannabinoid testing has no value. It means the result must be interpreted as postmortem evidence. It should not be forced into an antemortem model by relabeling a centrifuged autopsy fraction as ordinary serum.
For related principles, see Postmortem Redistribution and Postmortem Blood Sampling Sites and Interpretation.
Conversion is not measurement
If a laboratory measures whole blood and then multiplies by 1.6, the resulting plasma value was not measured. It was estimated using an assumed ratio. The distinction should remain visible in the report and testimony.
A proper interpretation should identify:
- The specimen type actually received
- Whether the sample was antemortem or postmortem
- Whether plasma or serum was physically separated
- The method used to separate it
- The measured result in the tested matrix
- Any conversion factor applied
- The scientific source and uncertainty for that factor
- Whether the downstream model was validated for the same matrix and population
If a published model was developed with plasma concentrations, direct substitution of whole-blood data can produce a false appearance of equivalence. Converting first may reduce the mismatch, but it does not erase biological and analytical uncertainty.
Why timing models require the right input
Some cannabinoid models attempt to estimate elapsed time since use from THC concentration or a metabolite-to-THC ratio. The mathematical output depends on the input matching the model's original definitions.
A plasma model expects plasma. A whole-blood result is not plasma merely because both are reported in nanograms per milliliter. A postmortem supernatant is even farther removed because death changes the specimen itself.
Giroud and colleagues observed that a metabolite-to-THC ratio might be less sensitive to matrix conversion if both analytes partition identically. But that premise must be established, not assumed. Analytical limits, instability, unequal partitioning, and postmortem changes can affect the numerator and denominator differently.
See Pharmacokinetics in Toxicology and Measurement Uncertainty in Toxicology.
The bottom line
The specimen matrix is part of the result.
The 2001 study found a relatively consistent plasma-to-whole-blood relationship in a small group of living cannabis users and suggested a study-based factor of approximately 1.6, used with caution. Its postmortem cases showed a higher and more scattered relationship. The authors did not advocate using postmortem cannabinoid concentrations to estimate the time of cannabis exposure through the same models.
The safest reporting practice is transparent: identify what was measured, identify what was estimated, and do not carry an antemortem conversion into a postmortem specimen without validation.
Source
Christian Giroud and colleagues, "Delta-9-THC, 11-OH-Delta-9-THC and Delta-9-THCCOOH plasma or serum to whole blood concentrations distribution ratios in blood samples taken from living and dead people," Forensic Science International 123(2-3) (2001) 159-164. PubMed record. DOI: 10.1016/S0379-0738(01)00538-2.
Editorial note: This article paraphrases the published study. Do not upload or reproduce the source PDF, tables, figures, or publisher-formatted pages.