Behind the Mask: Detection and Characterization of Benzalkonium Chloride Adulteration in Immunoassay-Based Urine Drug Testing for Tetrahydrocannabinol (THC).

Urine drug testing for tetrahydrocannabinol (THC) is widely performed using lateral flow immunoassay (LFI) kits. However, these tests are vulnerable to adulteration by benzalkonium chloride (BAK), a cationic surfactant that is a common ingredient in consumer products, such as eyedrops and antiseptics, which can produce false-negative THC results. BAK is believed to act by forming micelles around the THC metabolite 11-nor-9-carboxy-tetrahydrocannabinol (THC-COOH), blocking detection. Because BAK is not routinely included in specimen validity testing panels, it remains a significant potential masking agent in urine drug screening. This study evaluated BAK-containing products and confirmed that readily available commercial formulations can effectively mask THC in the urine. For example, when 25 mL of an antiseptic liquid containing BAK at a nominal concentration of 1300 ppm was added to 75 mL of urine, the LFI test was found to flip from true positive to false negative. Consumer products containing lower concentrations of BAK, such as eye drops, typically mask THC detection when added to urine at an equal volume. To address the vulnerability of LFI-based THC testing to BAK addition, a rapid, low-cost, noninstrumental screening approach based on the standard addition method was developed to distinguish true negatives from BAK-induced false negatives. In blinded testing of 45 urine samples, consisting of 15 true positives, 15 true negatives, and 15 false negatives, the method correctly identified all of the adulterated samples. These findings highlight a practical strategy for improving the reliability of THC urine drug screening in settings where LFI kits are routinely used.