THCA Was Excluded From a Blood Assay When Its Internal Standard Did Not Match
A potentially useful cannabinoid marker was left out of a validated whole-blood method because matrix effects prevented acceptable performance. The example shows why a promising biological target is not automatically a reportable laboratory analyte.
A broad extraction method with one important exclusion
The investigators developed disposable-pipette extraction and LC–MS/MS for ten cannabinoids and metabolites in 200 microliters of whole blood. They also attempted to include tetrahydrocannabinolic acid A, abbreviated THCAA in the paper.
Other included targets met the method’s validation criteria. THCAA, however, showed substantial and inconsistent ion suppression, with accuracy and precision outside acceptable bounds during validation.
An internal standard did not necessarily compensate
The investigators used a nearby-eluting labeled THCCOOH standard as a possible substitute because a matched labeled THCAA standard was not available to them. The substitute experienced much less suppression than THCAA itself.
Consequently, normalizing the analyte to that standard did not reliably correct the matrix effect. Variation among blood lots further restricted reproducibility. The authors excluded THCAA rather than representing it as successfully validated alongside the other targets.
What the failed inclusion teaches
This was a method-development result, not evidence that THCA can never be measured in blood. Another method using different preparation, separation or a suitable matched standard may perform differently.
For a case review, identify the exact analyte panel and the laboratory’s own validation rather than assuming that every cannabinoid named in a paper’s introduction was ultimately included. Analytical failure of one target also does not invalidate successful validation of the others. Biological interest, chemical detectability and reliable quantitative reporting remain separate steps.
Related: Interpreting cannabinoid measurements.
Primary research
Quantification of Cannabinoids and their Free and Glucuronide Metabolites in Whole Blood by Disposable Pipette Extraction and Liquid Chromatography Tandem Mass Spectrometry. Journal of chromatography. A (2016).
