723 - Cross-reactivity of labetalol with fentanyl on urine immunoassay

Poster Number: 723
Publication Number: 723.244

Maheen Farooq, Staten Island University Hospital, Brooklyn, NY, United States; Jonathan Blau, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell, Staten Island, NY, United States; Priyadarshani Giri, Staten Island University Hospital, Staten Island, NY, United States; Philip Roth, Staten Island University Hospital/Northwell Health, Staten Island, NY, United States

Background: Universal drug screening for substance use in delivering mothers aims to eliminate observer bias noted in targeted screening and identify neonates at risk for opiate withdrawal (NOWS). In response to the high rate of NOWS being seen in newborns, Staten Island University Hospital implemented universal urine drug screens (UDS) in 2018 on all laboring mothers admitted to Labor and Delivery, with a fentanyl immunoassay being added to the UDS panel subsequently. A shortcoming of any screening assay is the impact that a false positive may have on clinical decision making and the patient-medical team relationship. 

Objective: This study aims to determine the incidence of false-positive fentanyl immunoassay results and factors that can contribute to it as a part of the universal UDS. Labetalol, a commonly prescribed anti-hypertensive agent in pregnancy, can potentially cross-react with amphetamines on UDS. In this study, we aim to determine labetalol's cross-reactivity with a fentanyl test on the Sefria immunoassay. 

Design/Methods: This cross-sectional study uses maternal data gathered from the EMR. Mothers admitted to L&D from 2/1/18-10/31/22 after the addition of fentanyl immunoassay to the UDS who had a history of labetalol use were included. Parameters such as maternal age, gravida and para status, mode of delivery, maternal medications, UDS results, and neonatal meconium results were collected from the EMR. Mothers whose UDS were collected after administration of epidural (containing opioid analgesia), or whose UDS could not be collected, were excluded. Meconium of neonates whose mothers tested positive for fentanyl was sent for confirmatory testing via definitive liquid chromatography/tandem mass spectrometry (LC/MS). 

Results: 131 mothers receiving labetalol were admitted to L&D during the study period, of whom 19% (N=25) tested positive for fentanyl on the UDS immunoassay. The meconium of these 25 infants was sent for confirmatory testing with 0 (0%) testing positive for fentanyl. In 1 case, a maternal confirmatory fentanyl was positive, whereas the meconium in that case was incidentally cancelled.

Conclusion(s): This study showed a relatively high-rate false positive rate (18.3%) of the fentanyl immunoassay in mothers receiving labetalol. Mothers aged >36 appear to be at increased risk for a false positive result. Knowledge about the assays used to screen for any substance and the potential false positives and negatives associated is essential to effectively use any screening tool. Evidence of cross-reactivity of labetalol and fentanyl provides valuable information regarding limitations of this assay.