The research was initiated after scientists noticed a spike in false positive results for THC, the metabolite of the psychoactive chemical found in cannabis, during routine urine tests, with commonly used baby washes blamed for the faulty test results.

The researchers, from the University of North Carolina School of Medicine say they don't know what is causing the faulty results, but did find some of the detergents in the skin care products to blame.

The study, led by Catherine A. Hammett-Stabler, professor of pathology and laboratory medicine at UNC Hospitals, suggests the accuracy of routine urine drug testing of newborns to identify exposure in the womb to marijuana may be interfered with by chemicals in commonly used baby soap and wash products.

It appears in the June issue of Clinical Biochemistry, ​with scientists stating that accurate identification of illicit drug exposure in newborns "is critical to ensure both protection of the child and support for the family."​

Revised screening protocol​

A key factor that helped initiate the study was a revised screening protocol set in place in February 2011 that increased the use at UNC Hospitals of the urine immunoassay for identifying tetrahydrocannabinol-delta 9-carboxylic acid (THC) in newborns.

The revised screening protocol was designed to fall in line behind the latest recommendations for newborn drug screening.

According to Hammett-Stabler, the clinical labs received a call in July 2011 from nurses in the Newborn Nursery asking questions about an increase in THC screens testing positive.

A review of testing from this unit since the previous January showed that 22 of the 120 urine drug screens performed were positive for THC, but none had undergone confirmation testing.

After a review of the mothers' and newborns' drug histories did not reveal any potential agents that could alter the screening test, a number of newer samples that had tested positive to THC screening were sent to a reference lab for confirmation with more sophisticated techniques, such as mass spectrometry.

"The results did not match up with the original screens; they were not confirmed,"​ Hammett-Stabler said. "This led to a meeting with the nurses and other key clinical staff to talk through how the samples were collected and what happens between baby delivery and the collection of urine samples."​

Zoning in on the culprit​

This led to an examination of all nursery-specific products that could potentially come in contact with urine samples and interfere with screening results: cotton balls, collection containers, possible dyes present in the outer portion of the diapers, before baby wash was identified.

The team then bought a range of baby washes and acquired some of the chemicals used to make up the baby washes including all that were in the products that gave the positive response.

"Our findings in this study drive home the point that confirmation by more sophisticated methods such as mass spectrometry should be considered before moving ahead with interventions such as child social services or child abuse allegations, which may be false, " ​said co-author Carl J. Seashore, associate professor of pediatrics at UNC.