An 83% Hair-Screen Confirmation Rate Was Conditional on a Positive Screen
The documented-user hair study confirmed THC-COOH in 19 of 23 screen-positive specimens. That 83% figure was not the percentage of all cannabis users detected by the testing process.
The denominator was selected by screening
The immunoassay used a cutoff of five picograms THC equivalents per milligram of hair. Positive screening results were then compared with THC-COOH confirmation at a different threshold and with a different analytical method. Four of the twenty-three positive screens did not meet that metabolite confirmation criterion.
Confirmation and overall detection are different measures
A high confirmation proportion among positive screens can coexist with missed exposure among screen-negative specimens. All participants came from a documented-use research program, and the paper also found cannabinoid-negative specimens. The conditional confirmation percentage cannot therefore be substituted for sensitivity among everyone who used cannabis.
Retain both targets and thresholds
The screening signal was expressed in THC equivalents, while confirmation identified a specific metabolite. A discrepancy does not automatically establish that every initial signal was analytically false; another cannabinoid could contribute. The defensible account states which specimens entered the calculation and what each test required. It does not turn a positive-screen confirmation rate into an individual use probability or an impairment measure.
Related analysis: the study’s broader findings and limitations.
Research cited
Huestis and colleagues (2006). Cannabinoid Concentrations in Hair from Documented Cannabis Users.
