A Postmortem THC Assay’s Quantification Limit Was 25 ng/mL
A validated method can still be unsuitable for assigning an exact value to a low-concentration specimen. One postmortem GC-MS study reported a whole-blood THC quantification limit of 25 ng/mL, an essential qualification when interpreting its case results.
Blood and urine had different targets
The method measured parent THC in postmortem whole blood and THC-COOH in hydrolyzed urine. It used liquid-liquid extraction, chemical derivatization and single-quadrupole GC-MS. These are different analytes in different matrices, rather than two interchangeable measurements of “cannabis level.”
The main validation table reported a 15 ng/mL blood THC detection limit and a 25 ng/mL quantification limit, with the quantitative range extending to 300 ng/mL. The urine THC-COOH quantification range was 50–300 ng/mL. The paper contains some inconsistent descriptions of detection and calibration limits, making the specific validated target and table particularly important.
Detection and exact reporting are separate claims
A detection limit addresses whether a signal can be distinguished under specified conditions. A quantification limit also addresses acceptable numerical performance. Being detectable below the quantitative range does not automatically validate an exact reported concentration there.
Likewise, a result below this method’s detection capability would not establish that no THC was present. Nor could this validation certify the accuracy of a different laboratory’s assay with a much lower reporting limit; that laboratory needs its own supporting data.
The case series was selected
The application involved 30 cases selected through positive urine immunoassay results, followed by examination of corresponding blood. That design demonstrates application to selected postmortem specimens. It cannot establish cannabis prevalence among all deaths or the probability that cannabis caused death. Other drugs were present in many listed cases.
The authors also lacked reference standards to challenge the method against other THC isomeric metabolites. This limits the scope of the interference evaluation. Accurate case interpretation therefore requires the validated range, compound identification, specimen history and independent cause-of-death evidence—not merely the statement that GC-MS was used.
Related: interpreting THC concentrations separately from impairment.
Primary research
Enhancement and validation of a quantitative GC–MS method for the detection of ∆9-THC and THC—COOH in postmortem blood and urine samples. DOI: 10.1016/j.mex.2024.102962.
