B-284 Evaluation of Bupivacaine Metabolite Interference With Norfentanyl in a Liquid Chromatography Tandem Mass Spectrometry Assay in Neonatal Urine

Abstract Background Bupivacaine is a local anesthetic commonly used when administering epidurals during labor and delivery. Bupivacaine metabolite, N-desbutylbupivacaine (NDB), has recently been shown to cause isobaric interference in common urine norfentanyl quantifier and qualifier multiple reaction monitoring (MRM) transitions. This can interfere with the detection of prenatal fentanyl exposure in this vulnerable patient population, which is especially concerning if fentanyl is unexpected. A recent review of urine fentanyl reporting at our institution identified that the number of urine norfentanyl results reported as an interference comment is increasing. A chromatographic review determined this is due to the presence of an unidentified peak co-eluting just before norfentanyl in both the qualifier and quantifier MRM transitions. Most of these samples are from neonates. This study aimed to determine if NDB is the cause of the interfering peak in the urine norfentanyl MRM transitions and evaluate if other specimen types are also affected. Methods NDB standards were prepared and tested using the routine urine fentanyl assay performed by LC-MS/MS in our laboratory, using norfentanyl MRM transitions 233.1 --> 84.1 and 233.1 --> 150.1. Chromatography results were compared to patients with the interference. A retrospective data pull was performed to identify neonatal patients with a urine fentanyl result who also had an umbilical cord or meconium testing requested. Referring centers with patients with the interference comment were contacted to determine neonatal urine collection practices and if bupivacaine was used in their labor and delivery departments. Results NDB produced the same chromatographic pattern observed in the patient samples with norfentanyl interference. This confirmed NDB as the source of the interference. Centers with high rates of NDB interference were contacted and confirmed bupivacaine was routinely administered during labor and delivery at their facilities. Retrospective data analysis identified 577 urine samples with either paired umbilical cord (547) or meconium (30) samples. Of those, 478/577 (82.8%) urine samples displayed NDB interference, and norfentanyl could not be reported due to interference; however, 59 samples were positive for parent fentanyl. Of the 478 urine samples with NDB interference, 455 had a matching cord sample, and 23 had a matching meconium sample. Further examination identified 34/455 (7.5%) cord samples with matching urine samples that had NDB interference were positive for fentanyl. This suggests umbilical cord samples may be useful in some cases to resolve whether a neonate was exposed to fentanyl if NDB interference is observed in urine. Of the 23 paired urine/meconium samples, no positive meconium samples were associated with a urine sample with NDB interference. Conclusions Bupivacaine metabolite is commonly seen in the urine of neonates if bupivacaine is administered as an epidural. Chromatographic separation of bupivacaine metabolite from norfentanyl should be evaluated, especially for centers that receive neonatal samples.