Evaluation of 4-fluoroisobutyrylfentanyl in blood samples from 247 authentic cases submitted to the New York City Office of Chief Medical Examiner in 2017–2018



4-Fluoroisobutyrylfentanyl (FIBF) was first identified at the New York City Office of Chief Medical Examiner (OCME) in May 2017 and was reported qualitatively due to the constant changes in “fentalogs” (analogs of fentanyl) identified. However, by the year’s end, FIBF was the fifth most common non-methadone synthetic opioid detected and a quantitative method was developed to better understand the significance of this compound.


A full quantitative validation was performed utilizing liquid chromatography–tandem mass spectrometry (LC–MS/MS). All cases submitted between May 2017 and December 2018 that tested positive for FIBF qualitatively were chosen for quantitation. In addition to compiling FIBF concentrations, user demographics and concurrent drug use were also investigated.


There were 247 FIBF-positive cases that were tested with blood concentrations ranging from < 0.1 to 331 ng/mL. The most commonly detected drugs in conjunction with FIBF were other opioids, benzodiazepines, cocaine, cannabinoids and ethanol. The most frequent users were male, White, and between 35 and 54 years of age. The concentrations of FIBF varied widely and showed no clear distinction when considering various case types. Although most often used in tandem with other drugs of abuse, the danger of this compound was demonstrated by the fact that FIBF was also reported as the sole intoxicant responsible for death.


The development of a quantitative method for FIBF has been beneficial at the OCME due to the significant number of positive cases reported and the current lack of sufficient published information on this analog. To our knowledge, there are only two papers that include FIBF concentrations in four cases; this article gives the largest number (247 cases) of FIBF levels in blood samples of its users to aid in the interpretation and analysis of significance of this compound.

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Correspondence to Justine Pardi.

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Pardi, J., Toriello, A. & Cooper, G. Evaluation of 4-fluoroisobutyrylfentanyl in blood samples from 247 authentic cases submitted to the New York City Office of Chief Medical Examiner in 2017–2018. Forensic Toxicol 38, 340–351 (2020). https://doi.org/10.1007/s11419-019-00518-6

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  • 4-Fluoroisobutyrylfentanyl
  • FIBF
  • Synthetic opioids
  • Fentanyl analogs
  • Whole blood samples
  • LC–MS/MS