THC TESTING FOUNDATIONS | RESEARCH EXPLAINED
The short answer: A THC blood result depends on what happens before the sample reaches the instrument. A 2018 study of QuEChERS extraction illustrates why recovering a compound, measuring it accurately, and interpreting its significance are three different tasks. The method’s name is not a certificate of reliability, and its detection limit is not an impairment threshold.
AI-assisted educational analysis of the cited research. The complete main paper, references, three figures and three tables were reviewed for this article. No personal review or authorship by Okorie Okorocha is asserted. This is a methods explanation, not a laboratory operating procedure or case opinion.
QuEChERS abbreviates quick, easy, cheap, effective, rugged and safe. In the procedure studied by Michal P. Dybowski and Andrzej L. Dawidowicz, extraction with acetonitrile and inorganic salts was followed by cleanup using a C18 sorbent. The prepared extract then underwent evaporation and chemical derivatization before gas chromatography-tandem mass spectrometry, or GC-MS/MS.
Those are separate operations. Extraction moves the target compounds into a fraction the laboratory can analyze. Cleanup reduces material that is not wanted in that fraction. Derivatization changes the chemical form measured by the GC-MS/MS procedure. The final chromatogram cannot, on its own, show that every preceding step performed acceptably.
The 2018 full-text study examined THC, 11-hydroxy-THC and 11-carboxy-THC, commonly called THC-COOH. It combined method-development experiments with application to specimens from 30 people suspected of drug-impaired driving who had positive cannabinoid saliva screening results. This was an analytical-method study, not a controlled experiment measuring driving ability.
Figure 3 is the central teaching example. The researchers varied the amounts of sodium chloride, magnesium sulfate, acetonitrile, C18 sorbent and blood. Recovery changed with those choices, and THC and its two metabolites did not always respond in the same way.
The sorbent experiment did not show a simple pattern in which adding more cleanup material steadily improved recovery. The blood-mass experiment likewise did not show that a larger specimen automatically produced better recovery. Each comparison kept the other specified conditions fixed. These plots describe that experimental system; they do not establish universal optimal settings for another laboratory.
The practical lesson is narrower than a recipe: changing a sample-preparation condition can change the analytical behavior of the target compounds. A laboratory cannot assume that a modification is harmless because the instrument and the name of the extraction technique remain unchanged.
Recovery asks how much target material is obtained through preparation, relative to the comparison used in the experiment. Accuracy asks how closely the reported measurement agrees with the reference value. Precision asks how consistently repeated measurements agree with one another.
Table 2 reports recovery above 55% for the three compounds under the selected conditions at 25 ng/g. It separately reports accuracy and precision results. The recovery statement does not mean that the method was only 55% accurate, nor does it justify multiplying a case result by a recovery correction taken from this paper.
The authors used a separate deuterated internal standard for each target compound. These labeled compounds were introduced before extraction and used as analytical comparators. That design matters because a measurement must account for the behavior of the analyte through the procedure. The presence of an internal standard still does not excuse a failed control or establish that all possible interferences have been excluded.
For the general concept, see the companion library’s explanation of extraction recovery. This article supplies a specific THC example rather than replacing that broader guide.
The authors estimated detection and quantification limits using signal-to-noise criteria. Table 2 lists a THC detection limit of 0.011 ng/g and a quantification limit of 0.033 ng/g. The methods section lists the lowest THC calibration level as 0.1 ng/g.
These numbers should not be silently treated as interchangeable. A signal-to-noise estimate below the lowest listed calibrator is not, by itself, a demonstration of accuracy and precision at that lower concentration. A reader evaluating low-level reporting should ask for the laboratory’s actual calibration range, low-concentration validation data and reporting rules. The paper is not a substitute for those records.
Keep the units attached to the values. Table 2 and the calibration description use ng/g, a mass-based concentration. Some validation prose uses ng/mL. This article preserves the table’s ng/g units and does not assume that the two units are numerically interchangeable.
The paper describes precision and accuracy experiments on five independent samples at a stated concentration of 25 ng/mL, assessed within a day and across three days. It also describes selectivity checks using blank blood from five volunteers. Those are defined experiments, not proof of identical performance at every concentration or in every specimen condition.
For matrix effects, the authors compared calibration slopes and reported no statistically significant difference. That supports a statement about the comparison performed. It does not establish that no blood specimen could ever affect the measurement.
There is also a collection detail worth resolving before transferring the procedure: the article calls the specimens whole blood but describes a collection system containing a coagulation activator. This article does not resolve that wording or turn it into a recommendation for specimen collection. The relevant laboratory must establish what material it tested and how its own collection and preparation were validated.
Table 3 shows application of the method to authentic specimens. It does not report a controlled dosing experiment or a measured driving-performance endpoint. Applying an assay to these specimens demonstrates a different thing from validating a concentration that establishes impairment in an individual.
The discussion includes inferences about time since consumption and refers to European legal norms. This guide does not adopt those statements as a current legal rule, a universal THC cutoff, or a reliable clock for an individual person’s last use. No safe-driving waiting time can be derived from this analytical study.
For a different sample-preparation problem, our CBD-to-THC derivatization guide explains a specific chemical-conversion experiment. It is not evidence that the same conversion occurred in this QuEChERS study.
Dybowski MP, Dawidowicz AL. Application of the QuEChERS procedure for analysis of Delta-9-tetrahydrocannabinol and its metabolites in authentic whole blood samples by GC-MS/MS. Forensic Toxicology. 2018;36:415-423. DOI: 10.1007/s11419-018-0419-8. PubMed record.
This is a historical method-development example, not a claim that this is the newest or best available assay. The publisher, PubMed, PMC and Crossref records checked on September 29, 2026 UTC did not identify a correction, retraction or expression of concern. No supplementary-material entry was located in the full-text record. That records check is not a guarantee that no later notice will appear.
This original explanation paraphrases the cited paper; its PDF, figures and individual case rows are not reproduced here. AI assistance and the absence of article-specific personal expert review are disclosed above. Educational information does not replace laboratory-specific validation or review of the complete record. Return to the resource library.