Griswold MK, et al. - This work compared the detection of fentanyl in oral fluid and urine specimens using liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) in a population of heroin users presenting to the Emergency Department after overdose. In this study, a detectable fentanyl exposure was evident among a strikingly high proportion of individuals. Using LC-QTOF-MS, the agreement between paired oral fluid and urine testing for fentanyl detection indicated a role for oral fluid testing in surveillance for nonpharmaceutical fentanyl. Additionally, LC-QTOF-MS use allowed for the detection of other clandestine opioids (acetylfentanyl and U-47700) in oral fluid.