What Drugs Cause Fentanyl False-Positives?
In our latest question and answer, the pharmacist discusses which drugs are known to cause false-positives for fentanyl.
What drugs can show up as a false-positive for fentanyl on an oral swab or urine test?
While urine drug screenings are notorious for their high rate of false-positives (i.e. testing positive for a drug that isn't actually present in a sample), false-positive results for fentanyl are less common.
Why Is Fentanyl Less Likely To Have A False-Positive Result?
Fentanyl is a fully synthetic opioid and is sometimes even described as a 'designer' opioid.
It is structurally distinct enough from other opioids that it isn't reliably detected on standard opioid urinalysis tests, and, if testing for it is necessary, specific tests need to be ordered.
One study, Laboratory Testing for Prescription Opioids, discusses this about fentanyl:
Other prescription opioid drugs such as fentanyl and buprenorphine are sufficiently distinct in structure compared to morphine that these drugs show essentially no reactivity in commonly marketed morphine-specific opiate immunoassays. Detection of these opioid drugs therefore requires entirely separate immunoassays that are specific for these compounds or methods capable of their detection and specific identification such as liquid chromatography–tandem mass spectrometry.
Why Do False-Positives Happen?
Even though a false-positive result for fentanyl isn't common, it has still been reported to happen, and this is simply due to how most common drug tests work.
Essentially, urine and oral swab drug tests use antibodies to react to a specific drug compound. When the drug compound being tested for is present in a sample, an antibody binds to it and produces a reaction. This is recorded as a positive result.
Unfortunately, in both oral drug swabs and urine screenings, antibodies can sometimes bind to the wrong substance and react. This is what causes a 'false-positive' result.
Most commonly, when an antibody binds to the wrong substance, the molecular structure is similar to what is being tested for.
When it comes to fentanyl, as mentioned above, it is fairly unique and therefore isn't often associated with false-positive reactions.
Common Drugs Causing False-Positives For Fentanyl
While false-positive tests for fentanyl don't happen often, several case studies have been published identifying drugs that may be responsible for false-positive results.
The four drugs with the most evidence for causing false-positives for fentanyl are:
- Risperdal (risperidone)
- Invega (paliperidone)
- Fanapt (iloperidone)
Risperidone and related compounds (e.g. paliperidone, iloperidone) have been extensively reported to be responsible for causing false-positive drug tests for fentanyl.
In fact, risperidone is listed as a potential drug to be aware of in educational bulletins and product information pamphlets for drug test products.
What To Do If A False-Positive Occurs
If you believe you have mistakenly received a false-positive result for fentanyl, you should request a more accurate confirmatory test.
Urine and oral swab drug tests should ideally only be utilized as an initial screening since they are subject to false-positives.
Other, more accurate tests, like GC-MS (gas chromatography-mass spectrometry), can detect specific compounds in a given sample, and don't rely on antibody reactions for results.
Answer SummaryTrazodone, Risperdal (risperidone) Invega (paliperidone) and Fanapt (iloperidone) have been reported to cause false-positive results on urine and oral swab drug screenings for fentanyl. However, false-positive results for fentanyl on these screenings aren't common due to its unique molecular structure.
- Urine Drug Screening: Practical Guide for Clinicians. Mayo Clinic Proceedings
- Toxicologic Testing for Opiates: Understanding False-Positive and False-Negative Test Results. PubMed
- False positive test dose and epidural fentanyl. PubMed
- Cross-reactivity of acetylfentanyl and risperidone with a fentanyl immunoassay. PubMed
- A Difficult Challenge for the Clinical Laboratory: Accessing and Interpreting Manufacturer Cross-Reactivity Data for Immunoassays Used in Urine Drug Testing. PubMed
- Laboratory Testing for Prescription Opioids. PubMed
- Providence Health Care St. Paul’s Hospital & Mount Saint Joseph Hospital Laboratory Medicine Bulletin
- False-Positive Interferences of Common Urine Drug Screen Immunoassays: A Review. Journal of Analytical Toxicology