Surprising source of positive medical lab test results was discovered by a special team including pathologists, medical technologists, nurses, and physicians
Some innovative sleuthing by clinical laboratory professionals at University of North Carolina School of Medicine (UNCSM) hospitals has helped solve a marijuana mystery involving neonatal screenings. An unexpected spike in “false positive” cannabis exposure screening results in newborns at the facilities triggered a study by UNCSM scientists.
Revised Screening Protocol Leads to Jump in False Positives
According to a story in MedCity News , in July 2011, the UNCSM clinical laboratories received a call from nurses in the neonatal nursery. They had noticed an increase in positive results in screenings for tetrahydrocannabinol-delta 9-carboxylic acid (THC). THC is the principal psychoactive component of the cannabis plant.
Five months earlier, in February 2011, a revised screening protocol had been instituted at UNC Hospitals, consistent with the latest recommendations for newborn drug screening. The revised protocol increased the use of the urine immunoassay for identifying THC in newborns, the story stated.
Urine samples were testing positive for THC. However, subsequent testing of meconium samples from the same newborns tested as negative. Neither the maternal nor infant drug histories showed agents that could alter screening results. Something other than marijuana use was causing the false positives.
Study Identifies False Positive Triggers Affecting Clinical Lab Test Results
“Mixtures of drug-free urine with various commercial products and materials that commonly contact newborns in our nursery were prepared and tested using the immunoassay screening methods in our laboratory,” the authors wrote in the abstract of the study, published in the June issue of Clinical Biochemistry.
The team started sending newer samples that tested positive for THC to a reference laboratory for confirmation using more sophisticated techniques, such as mass spectrometry, a story published at HealthCanal.com reported.
“The results did not match up with the original screens; they were not confirmed,” stated Catherine A. Hammett-Stabler, Ph.D., Professor of Pathology and Laboratory Medicine and Executive Director of the Core Laboratory of the William W. McLendon Clinical Laboratories at UNC Hospitals. He was the pathologist who led the study.
The multi-disciplinary study team consisted of clinical laboratory scientists, physicians, a social worker, and nurses at UNCSM and UNC Hospitals. They concluded that something was getting into the samples and interfering with the medical laboratory tests and began to evaluate the screening process for any pre-analytical or analytical sources of error or interference.
“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,” Hammett-Stabler explained.
Weeding out possible culprits, the researchers eliminated cotton balls, collection containers, and possible dyes in the outer portion of diapers. Then they began testing the newborn nurseries’ commercially available baby washes.
Testing showed that four out of seven commercial baby soaps tested caused a “false positive” in cannabis exposure screening results. They identified polyquaternium-11 and cocamidopropyl betaine as the chemicals potentially triggering the false positives, a story published at medicalxpress.com stated.
“Structurally, these things don’t look like marijuana metabolites, but clearly they are being recognized by the antibodies used in the immunoassay,” Hammett-Stabler said in a statement, according to MedCity News. “And when testing those reagents in increasing concentrations, we did see a positive response, albeit really low, just over the point of positive.”
Mass Spec Analysis Helped Identify Agent Causing False Positive Results
Citing important health and legal ramifications, the study authors stressed that accuracy of the results cannot be undervalued. “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,” stated study co-author Carl J. Seashore, M.D., Associate Professor of Pediatrics at UNC and Director of the Newborn Nursery.
Clinical laboratory managers and pathology groups should take notice of this potential source of false positive screening results. They may want to consider a more sensitive method of confirmation before initiating interventions in similar situations. It is also a reminder of the need for vigilance against sources of error that may derive beyond the walls of the clinical laboratory.
—Pamela Scherer McLeod