An Unidentified Serum Compound Does Not Justify Subtracting 14.6% From Every THC-COOH Result
A serum study found a compound that could interfere with THC-COOH measurement, but its reported median relative abundance was not a universal measurement error. The finding supports checking a laboratory’s separation and identification procedures. It does not justify subtracting 14.6% from every reported cannabis-metabolite result.
The problem appeared in authentic human serum
Toennes and colleagues were developing liquid chromatography–tandem mass spectrometry for THC, 11-hydroxy-THC and THC-COOH. A shoulder appeared beside the THC-COOH peak in specimens from cannabis users. It was absent from the drug-free and fortified specimens used in their earlier validation work.
The interfering compound shared the monitored mass transitions with THC-COOH, although its relative fragment intensities differed. Changing the chromatography separated the two signals. The researchers proposed a structurally related isomer, but did not establish the compound’s exact structure or biological origin.
Why the denominators are 98, 93 and 73
The investigators examined 98 forensic specimens. Four contained neither detected cannabinoids nor the interfering compound, and one contained THC-COOH below the lower quantification limit. That left 93 evaluable THC-COOH-positive samples, all of which showed the additional compound.
They excluded another 20 specimens from the relative-abundance analysis because the interfering signal was too low for satisfactory integration. The reported range of 4.5%–51.0%, with a median of 14.6%, therefore came from 73 specimens. It was not a population prevalence estimate or a result based on all 98 specimens.
The percentage was an estimate of relative abundance
The unknown compound had no independently characterized calibration standard in this study. To estimate its amount, the researchers used the THC-COOH calibration and the summed areas of two fragment signals. They expected that approach to help accommodate differences in fragment intensities.
Consequently, the percentages depended on an analytical assumption about the unknown compound’s response. They were not direct measurements of how much every laboratory overestimated THC-COOH. Any actual bias would also depend on whether the method separated the peaks, which transition it used for quantification and how it integrated the signal.
The optimized method provided an important counterexample
The researchers compared their optimized routine chromatography with a longer separation in the 93 samples. The mean difference in THC-COOH results was approximately 0.01%, with a relative standard deviation of 2.6%. That finding showed that a method could measure THC-COOH reliably despite the compound’s presence when appropriate separation and integration were used.
The study therefore supports a method-specific inquiry. It does not support a blanket claim that mass spectrometry overreports cannabis metabolites. The related issue of testing authentic specimens during cannabis-assay validation also matters when evaluating a laboratory’s evidence.
Keep the matrix and the forensic question separate
These were human serum measurements, not a controlled driving experiment or a validation in urine, saliva or hair. The interference concerned THC-COOH, a non-psychoactive metabolite; the reported percentages were not errors demonstrated for parent THC. Even a sound metabolite measurement does not establish impairment at an earlier event.
For an individual result, the useful records are the chromatograms, retention-time and ion-ratio criteria, integration boundaries and validation in authentic serum. The paper cannot establish a case-specific correction without those records.
Primary research
Stefan W. Toennes, Stephanie Hanisch, Werner Pogoda, Cora Wunder and Alexander Paulke. Analytical and Bioanalytical Chemistry (2015), 407:463–470; published online in 2014. Pitfall in cannabinoid analysis—detection of a previously unrecognized interfering compound in human serum.
