A false positive for fentanyl occurs when a drug test indicates the substance is present, even though fentanyl is not in the person’s system. These inaccurate results can lead to misdiagnosis in medical settings, legal issues, or personal distress. Understanding the factors that contribute to false positives is important for accurate interpretation of test results.
How Initial Fentanyl Tests Work
Initial fentanyl drug screenings commonly employ immunoassay techniques, such as rapid test strips or urine drug screens. These tests detect specific antibodies or antigens that bind to fentanyl or its metabolites. The methodology involves competition between the drug in the specimen and a labeled drug for antibody binding sites. These initial screening methods are designed to be quick, easy, and cost-effective for preliminary detection. However, their methodology also makes them susceptible to false positive results.
Common Substances Causing False Positives
Several common substances can lead to false positive fentanyl results on initial screenings. Medications are a frequent cause, including certain antipsychotics like quetiapine (Seroquel) and risperidone, as well as some antibiotics such as levofloxacin (Levaquin) and ofloxacin. Antihistamines, particularly diphenhydramine (found in Benadryl), have also been implicated in false positives for opioids, including fentanyl. Some antidepressants like trazodone, and certain beta-blockers such as labetalol, can also cause such results.
Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can also interfere with drug tests. Dietary factors or supplements can interfere with immunoassay tests. Sample contamination, from environmental factors or improper handling, can also lead to false positive outcomes.
Why False Positives Occur
False positives in immunoassay tests stem from cross-reactivity. This occurs when antibodies designed to detect fentanyl inadvertently bind to other molecules with a similar chemical structure, even if those molecules are not fentanyl. For example, risperidone and its metabolite 9-hydroxyrisperidone cross-react with fentanyl immunoassays. Similarly, fentanyl analogs like acetylfentanyl can also cross-react with these tests.
Metabolism of certain substances can also contribute to false positives. Some medications are processed into metabolites that resemble fentanyl or its own metabolites, such as norfentanyl, triggering a positive result. Initial screening tests prioritize sensitivity to detect even small amounts of a substance, making them less specific in differentiating between fentanyl and other compounds. This trade-off between sensitivity and specificity increases the likelihood of false positive results, as the test broadly looks for similar molecular shapes rather than definitively identifying fentanyl.
Confirming a Test Result
Following an initial positive screening result, confirmatory testing is an important step to ensure accuracy. The gold standard for confirming fentanyl presence is Gas Chromatography-Mass Spectrometry (GC/MS) or Liquid Chromatography-Mass Spectrometry (LC/MS). These advanced analytical methods are highly accurate and can precisely identify and quantify specific substances by analyzing their unique molecular structures.
LC/MS, particularly liquid chromatography-tandem mass spectrometry (LC-MS/MS), is widely used and offers excellent specificity and sensitivity for detecting fentanyl and its main metabolite, norfentanyl. Unlike initial immunoassays, GC/MS and LC/MS can differentiate between fentanyl and other compounds that might have caused a false positive on the preliminary screen. This confirmatory testing is important, as an initial positive result does not definitively mean fentanyl is present, and accurate verification is necessary for medical or legal actions.