AACC: Labs should be aware of false positives on THC drug screens

2019 08 05 17 59 6641 Aacc 2019 400

A study of immunoassays used for detecting tetrahydrocannabinol (THC) -- the psychoactive part of marijuana -- found that other types of cannabinoids have the potential to give a positive result, researchers reported on August 5 at the American Association for Clinical Chemistry (AACC) annual meeting in Anaheim, CA.

Cannabinol (CBN), which is marketed as an over-the-counter sleep aid, gave a positive result on one of two THC immunoassays tested in a study of urine samples taken at ARUP Laboratories, a nationwide clinical and anatomical pathology reference laboratory and a nonprofit arm of the University of Utah. Specifically, CBN gave a positive result on the Emit II Plus assay on a Beckman Coulter AU5810 instrument, but it did not give a positive result with the Microgenics MultiGent test on the Abbott Architect product, the researchers reported.

The researchers also tested three other types of cannabinoids in urine samples -- cannabidiol (CBD), cannabichromene (CBC), and cannabigerol (CBG) -- but did not identify the same cross-reactivity issues reported for CBN.

The study shows the unknowns for consumers and laboratories alike regarding effects of cannabinoids and risks for false positives in drug tests. The legalization of marijuana in some U.S. states and the first approval by the Food and Drug Administration of a prescription-grade formulation of CBD (GW Pharmaceuticals) for a rare form of epilepsy has given a stamp of legitimacy to cannabis-derived products. CBD may be stealing the headlines nowadays, but according to the University of Utah researchers, CBN is also commonly used and may affect lab results.

Cannabinoid products on the rise

The study presented at the AACC meeting was undertaken in recognition that some patients treated for pain management and taking CBD tested positive on THC tests, explained Kamisha Johnson-Davis, PhD, medical director of ARUP Laboratories and an associate professor of pathology at the University of Utah. Staff members wanted to see if the THC assays could detect CBD, as this is not indicated on the package inserts for the testing kits that were being used, she explained in an interview.

The researchers tested purified forms of cannabinoids, and it's unclear what the results would be for marketed products. CBD products are not heavily regulated and the purification process varies, so they could actually contain THC, which would give a positive reading on a drug test, Johnson-Davis suggested.

When doing screening and confirmatory tests, physicians and laboratory directors should be thinking about what other cannabinoid compounds a patient might be taking and the potential for cross-reactivity, said co-author Grace Kroner, PhD, a clinical chemistry fellow at the University of Utah.

"It's important to keep in mind false positives," she added.

As for patients, "if they do test positive for a drug and are using a CBD product, that is certainly something they should bring up with their physicians," she said.

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