What medications can cause false positive results for fentanyl on urine drug screens?

Medications That Can Cause False Positive Results for Fentanyl on Urine Drug Screens

Several medications can cause false positive results for fentanyl on urine drug screens, including risperidone, ziprasidone, and diphenhydramine at high concentrations. Understanding these cross-reactivities is crucial for proper interpretation of test results and avoiding potential negative consequences for patients.

Common Medications Causing False Positive Fentanyl Results

• Antipsychotics:

  • Risperidone (Risperdal) and its metabolite 9-hydroxyrisperidone have been confirmed to cross-react with fentanyl immunoassays 1
  • Long-acting injectable risperidone (Risperdal Consta) has been documented to cause false positive fentanyl results in multiple patients 2
  • Ziprasidone (Geodon) has structural similarities to risperidone and has been reported to cause false positive fentanyl results 3

• Antihistamines and Common Cutting Agents:

  • Diphenhydramine (Benadryl) can cause false positive results at concentrations at or above 1 mg/mL 4
  • This is particularly concerning as diphenhydramine is a common cutting agent in heroin 4

• Stimulants (when present in high concentrations):

  • Methamphetamine and MDMA (ecstasy) can trigger false positives at concentrations at or above 1 mg/mL 4

Understanding Drug Testing Limitations

• Preliminary screening tests (immunoassays) are more prone to false positives compared to confirmatory tests 5
• The window of detection for fentanyl in urine is relatively short, typically less than 30 hours 5
• Confirmatory testing using more specific methods like gas chromatography/mass spectrometry (GC/MS) should be used when results can reasonably be expected to affect patient management 6

Best Practices for Interpreting Fentanyl Test Results

• When unexpected positive results occur, consider a broad differential diagnosis before taking action 6
• Discuss unexpected results with the laboratory or toxicologist 6
• Be aware that false-negative results can also occur if the sample is adulterated or if a patient provides someone else's urine sample 5
• Understand that some laboratories perform rapid preliminary screening tests followed by confirmation tests for positive results 5

Important Considerations for Clinicians

• Many providers have inadequate training in interpreting urine drug test results, and incorrect interpretation can have serious consequences 6
• Establish a working relationship with your local toxicologist who conducts the assays and can consult on results interpretation 6
• Be aware that rifampin can cause cross-reactivity and false-positive urine screening tests for opiates when using the KIMS (Kinetic Interaction of Microparticles in Solution) method 7
• Consider that immunoassay positive results should be considered presumptive until confirmed by a second independent chemical technique 8

Preventing Misinterpretation

• Explain to patients that urine drug testing is intended to improve their safety 6
• When interpreting results, be familiar with the drugs included in the testing panels used in your practice 6
• Consider that biologic drug testing panels may not include all common drugs of abuse such as fentanyl, carfentanil, or buprenorphine 5
• Be aware that the consequences of a false-positive result can be severe in some contexts (e.g., loss of child custody, legal implications) 5