8-THC-COOH cross-reactivity with cannabinoid immunoassay kits and interference in chromatographic testing methods

Because of structural similarities, the presence of 11-Nor-9-carboxy-& UDelta;(8)-tetrahydrocannabinol (& UDelta;8-THC-COOH) in a urine specimen might interfere with testing for 11-Nor-9-carboxy-& UDelta;(9)-tetrahydrocannabinol (& UDelta;9-THC-COOH). A set of samples containing & UDelta;8-THC-COOH with concentrations ranging from 10 to 120 ng/mL were tested at cut-offs of 20, 50 and 100 ng/mL using cannabinoid immunoassay reagents from three different manufacturers. Cross-reactivities ranged from 87% to 112% for & UDelta;8-THC-COOH at the cut-off of 50 ng/mL for the three different platforms. Additionally, samples containing both & UDelta;8-THC-COOH and & UDelta;9-THC-COOH were fortified by the National Laboratory Certification Program (NLCP). U.S. Department of Health and Human Services (HHS)-Certified Laboratories tested the samples to determine the interference of & UDelta;8-THC-COOH on confirmatory tests commonly used in workplace drug testing laboratories for the confirmation and quantification of & UDelta;9-THC-COOH. When evaluating confirmation and quantification of & UDelta;9-THC-COOH in the presence of & UDelta;8-THC-COOH, unreportable results for & UDelta;9-THC-COOH were observed because of chromatographic interference or mass ratio failures. However, there were no false-positive & UDelta;9-THC-COOH reports from any HHS-certified laboratory.