Newborns test false positive for marijuana

Some newborns are testing falsely positive for marijuana exposure, which can lead to erroneous allegations of child abuse, researchers from the University of North Carolina, Chapel Hill, report.

They undertook their study in response to a query from the newborn nursery about screens for 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid (THC), the main chemical component of marijuana. The nursery had observed an increase in the number of urine screens positive for THC without confirmatory positive results from meconium screens.

To investigate the reasons for the false positives, the researchers mixed drug-free urine samples with commercial baby soaps and hand washes routinely used in the nursery and screened them using immunoassay methods. What they found is that several commercial baby soaps caused a measurable, dose-dependent positive response on the THC immunoassay. Foaming hand soap did not react with the assay.

Chemical surfactants in the baby washes that interfered strongly with screening results included polyquaternium 11, cocamidopropyl betaine, and a mixture of PEG 80, sorbitan laurate, and cocamidopropyl betaine.

Lack of a standard protocol for newborn urine collection complicated the study. The collection bag, gauze, or diaper was applied either before or after the baby was washed, and babies washed before collection were not rinsed using a reproducible technique. Nevertheless, the investigators note, the small amounts of soap used in the study could realistically reflect the amount of residual soap on the skin after washing.

The study highlights the need to confirm positive marijuana test results to rule out confounding factors such as surfactant contamination before involving child social services, which can have significant economic and legal repercussions on patient care, the researchers say. They recommend reflex confirmatory testing of all positive urine and meconium screening results using a more specific and sensitive test than the screening assay.

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