BACKGROUND: False-positive drug screen results for tetrahydrocannabinol (THC) have been observed. This study investigated the rate of unconfirmed positive screen results in infant and noninfant urine samples and evaluated possible reasons for differences. METHODS: The rate of unconfirmed positive THC screen results for urine samples was determined retrospectively in 2 independent data sets (n ϭ 14 859, reference laboratory; n ϭ 21 807, hospital laboratory) by comparing positive

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... drug screen results with the associated results of confirmation tests. We then assessed the rate of positive THC screens for samples with varying likelihoods of cannabinoid presence to evaluate the contribution of infant-specific urine constituents to positive results. Finally, a method to detect a THC metabolite (11-hydroxy-⌬ 9 -THC) that occurs in meconium was developed to determine its prevalence in infant urine. RESULTS: Positive screen results failed to confirm more frequently in samples from infants (47%) than in noninfants (0.8%). The hospital laboratory observed a similar discrepancy with a different immunoassay. Infant samples with a high likelihood of containing cannabinoids despite negative confirmatory results had a similar rate of positive screening results (50%, n ϭ 20), whereas all samples with a low likelihood of containing cannabinoids screened negative (n ϭ 23). 11-Hydroxy-⌬ 9 -THC was not detected in any infant urine sample tested (n ϭ 16). CONCLUSIONS: Conventional confirmatory methods for THC may be inappropriate for urine samples from infants. Our results suggest that one or more currently unrecognized THC-associated compounds are responsible for positive THC screen results for infant urine, as opposed to an infantassociated interference. c Data are presented as the median (range). d Only 8 of 14 samples testing positively in the urine screen had confirmatory testing performed. e Only 239 of 2788 samples testing positively in the urine screen were sent for confirmatory testing. Brief Communication