THC Blood Extraction: What QuEChERS Validation Establishes in a Forensic Sample
A validated extraction procedure can support the reliability of a THC measurement in whole blood. It cannot, by itself, establish when cannabis was used or whether a person was impaired. Those are separate interpretive questions requiring evidence beyond extraction performance.
What the laboratory study tested
Dybowski and Dawidowicz adapted QuEChERS, a sample-preparation approach, for measuring delta-9-THC, 11-hydroxy-THC and THC-COOH by gas chromatography with tandem mass spectrometry. The supplied paper describes authentic whole-blood samples from 30 people suspected of driving under the influence who had positive oral-fluid screening results.
The researchers investigated extraction conditions and cleanup, then evaluated analytical performance. That work matters because blood contains substances that can interfere with recovering or measuring a small amount of a drug. A preparation that performs well in a clean standard solution is not automatically adequate for authentic blood.
Recovery and accuracy answer different questions
Extraction recovery describes how much of an analyte the procedure retrieves under specified conditions. Calibration, precision and detection limits address additional aspects of measurement. A defensible result depends on the method as a whole, including its controls and the specimen actually tested.
The three measured compounds should also remain distinct. A reported parent-THC concentration is not interchangeable with a metabolite concentration. Nor should a method described for whole blood be assumed to produce directly interchangeable values in plasma or serum.
What the authentic samples do not validate
The case specimens demonstrate an application of the method. They do not constitute a study comparing blood results with independently measured driving impairment. Selection after positive saliva screening also means the sample is not a representative population for estimating the prevalence of cannabis use or a screening test’s false-positive rate.
When reviewing a laboratory report, useful questions include whether the validated matrix matches the submitted specimen, whether the result lies within the validated quantitative range, and whether identification and quality-control criteria were satisfied. Only after those measurement questions should an expert address the separate questions of cannabis exposure, impairment and crash causation.
Primary research
Application of the QuEChERS procedure for analysis of Δ9-tetrahydrocannabinol and its metabolites in authentic whole blood samples by GC–MS/MS. DOI: 10.1007/s11419-018-0419-8.
