THC Equivalents in a Hair Immunoassay Were Not a Direct Measurement of Parent THC

The hair-screening assay responded more strongly to THC-COOH than to parent THC. Its reported THC-equivalent signal should not be read as a chemically specific THC concentration.

Antibody response differed across cannabinoids

The study reported relative cross-reactivity of 340% for THC-COOH when parent THC was defined as 100%. Other cannabinoids also produced responses, while CBD had very low cross-reactivity in this assay. These were method-specific antibody responses, not relative psychoactive potencies.

A screening unit combines response and calibration

A signal expressed in THC equivalents means the response was interpreted against a THC-based calibration. It does not establish that all of that response came from parent THC. The paper’s estimated effective threshold for THC-COOH consequently differed from the numerical THC-equivalent screening cutoff.

Chemical confirmation answers a narrower question

GC–MS/MS separately measured parent THC and THC-COOH. That distinction matters when interpreting a positive screen, a confirmation discrepancy or an apparent concentration. The cross-reactivity figures should not be extended to another manufacturer’s assay without its own data. Nor do they allow a hair screening number to be converted into a blood concentration, dose or functional effect. The study illustrates why the calibration name and the actual substances recognized by the assay must remain separate.

Related analysis: the study’s broader findings and limitations.

Research cited

Huestis and colleagues (2006). Cannabinoid Concentrations in Hair from Documented Cannabis Users.