Clearance may take longer
In toxicology testing for fentanyl, clinicians should also note the important difference between fentanyl and other opioids – that fentanyl, because of its high lipophilicity, may be detected in urine toxicology testing up to 3 weeks after last use. This is much longer than the 2- to 4-day clearance observed with other opioids, possibly causing patients to continue to test positive for the drug weeks after cessation.
This effect was observed in one recent study of 12 opioid use disorder patients in a residential treatment program who had previously been exposed to daily fentanyl.
The study showed the mean amount of time of fentanyl clearance was 2 weeks, with a range of 4-26 days after last use.
The authors pointed out that the findings “might explain recent reports of difficulty in buprenorphine inductions for persons who use fentanyl, and point to a need to better understand the pharmacokinetics of fentanyl in the context of opioid withdrawal in persons who regularly use fentanyl.”
Though the study was small, Dr. Salsitz said “that’s not a stumbling block to the important finding that, with regular use of fentanyl, the drug may stay in the urine for a long time.”
Dr. Salsitz noted that similar observations have been made at his center, with clinicians logically assuming that patients were still somehow getting fentanyl.
“When we initially found this in patients, we thought that they were using on the unit, perhaps that they brought in the fentanyl, because otherwise how could it stay in the urine that long,” he noted. “But fentanyl appears to be more lipophilic and gets into the fat; it’s then excreted very slowly and then stays in the urine.”
Dr. Salsitz said most practitioners think of fentanyl as a short-acting drug, so “it’s important to realize that people may continue to test positive and it should be thought of as a long-acting opioid.”
Opiate screening tests don’t work
Dr. Salsitz warned of another misconception in fentanyl testing – the common mistake of assuming that fentanyl should show up in a test for opiates – when in fact fentanyl is not, technically, an opiate.
“The word opiate only refers to morphine, codeine, heroin and sometimes hydrocodone,” he explained. “Other opioids are classified as semisynthetic, such as oxycodone, or synthetics, such as fentanyl and methadone, buprenorphine.”
“In order to detect the synthetics, you must have a separate strip for each one of those drugs. They will not show up positive on a screen for opiates,” he noted.
The belief that fentanyl and other synthetic and semisynthetic opioids will show positive on an opiate screen is a common misconception, he said. “The misunderstanding in toxicology interpretation is a problem for many practitioners, [but] it’s essential to understand because otherwise false assumptions about the patient will be considered.”
Another important testing misreading can occur with the antidepressant drug trazodone, which Dr. Salsitz cautioned may falsely test as positive for fentanyl on immunoassays.
“Trazodone is very commonly used in addiction treatment centers, but it can give a false positive on the fentanyl immunoassay and we’ve had a number of those cases,” he said.
Dr. Salsitz had no disclosures to report.
The Psychopharmacology Update was sponsored by Medscape Live. Medscape Live and this news organization are owned by the same parent company.