The Analyte Behind a Positive Whole-Blood Cannabinoid Screen
A positive whole-blood cannabinoid screen in this inhalation study was not a direct quantitative measurement of parent THC. The ELISA used a THCCOOH-based cutoff and responded to more than one cannabinoid.
The screen and the quantitative assay measured differently
In seventeen adults who had not used cannabis in the preceding month, researchers collected whole blood after smoked and vaporized cannabis. The blood ELISA used a cutoff of 10 ng/mL for THCCOOH, a THC metabolite.
The paper separately reported manufacturer-described cross-reactivity, including response to parent THC. The accompanying liquid chromatography–tandem mass spectrometry analysis quantified THC, 11-OH-THC, THCCOOH and THCCOOH-glucuronide individually. A screening classification and those compound-specific concentrations were different outputs.
The cutoff label matters
Calling the screening cutoff “10 ng/mL THC” would change the analyte named by the method. Nor can a positive screen be converted directly into an exact concentration of parent THC when several compounds can contribute to its response.
Interpretation should retain the specimen matrix, target calibrator, cross-reactivity information and confirmation procedure. An oral-fluid or urine screen may have a different target and different analytical characteristics even when all are described informally as cannabis tests.
Detection and impairment remain separate
The study’s concentrations did not yield a single reliable indicator of behavioral impairment. The pharmacokinetic paper and its companion acute-effects report also describe the same participant experiment, not independent replications. The useful first question is what the positive screen actually represents before drawing conclusions about functioning.
Related analysis: blood and oral-fluid patterns after inhaled cannabis.
Research cited
Spindle and colleagues (2019), Acute Pharmacokinetic Profile of Smoked and Vaporized Cannabis in Human Blood and Oral Fluid. DOI: 10.1093/jat/bky104.
