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Test ID: FENTS    
Fentanyl, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Monitoring fentanyl therapy

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Fentanyl is an extremely fast acting synthetic opioid related to the phenylpiperidines.(1,2) It is available in injectable as well as transdermal formulations.(1) The analgesic effects of fentanyl is similar to those of morphine and other opioids;(1) it interacts predominantly with the opioid mu-receptor. These mu-binding sites are discretely distributed in the human brain, spinal cord, and other tissues.(1,3)


Fentanyl is approximately 80% to 85% protein bound.(1) Fentanyl plasma protein binding capacity decreases with increasing ionization of the drug. Alterations in pH may affect its distribution between plasma and the central nervous system. The average volume of distribution for fentanyl is 6 L/kg (range 3-8).(3,4)


In humans, the drug appears to be metabolized primarily by oxidative N-dealkylation to norfentanyl and other inactive metabolites that do not contribute materially to the observed activity of the drug. Within 72 hours of intravenous (IV) administration, approximately 75% of the dose is excreted in urine, mostly as metabolites with <10% representing unchanged drug.(3,4)


The mean elimination half-life is:(1-3)

-IV: 2 to 4 hours

-Iontophoretic transdermal system (Ionsys), terminal half-life: 16 hours

-Transdermal patch: 17 hours (range 103-22 hours, half-life is influenced by absorption rate)


  - Lozenge: 7 hours

  - Buccal tablet

       - 100 to 200 mcg: 3 to 4 hours

       - 400 to 800 mcg: 11 to 12 hours


In clinical settings, fentanyl exerts its principal pharmacologic effects on the central nervous system. In additions to analgesia, alterations in mood, euphoria, dysphoria, and drowsiness commonly occur.(1,3) Because the biological effects of fentanyl are similar to those of heroin and other opioids, fentanyl has become a popular drug of abuse.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Not applicable

Interpretation Provides information to assist in interpretation of the test results

Both fentanyl and norfentanyl are reported.


Tolerant individuals may require many-fold increases in dose to achieve the same level of analgesia, which can greatly complicate interpretation of therapeutic drug monitoring (TDMA) results and establishment of a therapeutic window.


Concentration at which toxicity occurs varies and should be interpreted in light of clinical situation.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

No significant cautionary statements

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Gutstein HB, Akil H: Chapter 21: Opioid analgesics. In Goodman and Gilmans The Pharmacological Basis of Therapeutics. Vol 11. Edited by JG LL Hardman, AG Gilman, New York, McGraw-Hill Book Company, Inc, 2006

2. Kerrigan S, Goldberger BA: Opioids. In Principles of Forensic Toxicology. Edited by B Levine. Second edition. Washington DC AACC Press, 2003, pp 187-205

3. Package insert: DURAGESIC (fentanyl transdermal system. Pharmaceutical Products, LP, 2006

4. Baselt RC: Disposition of Toxic Drugs and Chemicals in Man. Eighth edition. Foster City, CA. Biochemical Publications. 2008 pp 616-619