A Little Ditty...About Immunoassays.


I had a urine drug test and my doctor/probation officer/random person told me I was positive for amphetamines. Why did I test positive? I've never taken amphetamines in my life.


More than likely, you have have consumed an amphetamine drug at least once in your life. There are more drugs in the amphetamine/phenethylamine family than just amphetamine (Adderall) and illicit methamphetamine, typically known as "crystal meth". One very common drug in this class is pseudoephedrine, typically used as a nasal decongestant in medications such as Allegra-D, Mucinex-D, Sudafed, and Zyrtec-D.

But the first question that I would ask is, was the test an immunoassay screen? Immunoassay screening tests are based on the principle that antibodies are able to recognize and bind to the drug of interest. These antibodies are designed to be highly selective – meaning they preferentially bind to the drug of interest. In the absence of the drug of interest, this preferential binding does not eliminate the possibility of binding to other drugs that have similar chemical characteristics. This secondary binding is commonly referred to as a “false positive” result. Unfortunately, it is not possible to design an antibody that binds to a single drug exclusively. Additionally, given the myriad of drugs and drug metabolites, it is also not possible to evaluate all possible “false-positives.”

Historically, an amphetamines immunoassay  uses either amphetamine or methamphetamine as a target compound, but it is very susceptible to other cross-reacting substances leading to “false positive” screening results. The following drugs have been known to cross react with various amphetamine immunoassay tests:

Amantadine, Bupropion, Chloroquine, Chlorpromazine, Desipramine, Ephedrine, Fenfluramine, Labetalol, Mexiletine, Procainamide, Phentermine, Phenylephrine, Pseudoephedrine, Ranitidine, Trazodone (mCPP is formed as a metabolite)

 So, it is important to keep a detailed list of substances consumed over the past 7-10 days. Simple use of over-the-counter Zantac (ranitidine) for heartburn or GERD could lead to a positive immunoassay test for amphetamines. Or use of a nasal decongestant such as pseudoephedrine could lead to a positive immunoassay test for amphetamines. The use of prescription medication Adipex (phentermine) could easily cause a positive immunoassay test for amphetamines.
The second question I would ask is, was a confirmatory test completed? While the screening test can be valuable for interpretation of urine toxicology results, especially in an emergency medicine situation, the possibility of “false positive” results is the primary reason for submitting the specimen to a laboratory for confirmatory testing. The laboratory confirmation testing utilizes either gas chromatography (GC) or liquid chromatography (LC) coupled to mass spectrometry (MS). A properly validated confirmatory test is not susceptible to the “false positive” results associated with immunoassay screening techniques. The mass spectrometric analysis provides what is effectively a “chemical fingerprint” pattern that is unique for each drug.  IF a confirmatory test was not completed, request it! While the scope of an assay is highly dependent on the individual laboratory doing the analysis, from my experience, the routine confirmatory urine amphetamines test only monitors amphetamine, MDA, MDMA, and methamphetamine. Some labs offer an expanded amphetamines panel and may include compounds such as ephedrine, MDEA, and pseudoephedrine or even other substituted cathinones such as MDPV and methylone.

If one tests positive for amphetamine and/or methamphetamine by a mass spectrometry- based method, then one has been exposed to or consumed a drug that either contains amphetamine and/or methamphetamine or metabolizes to either drug. It is also important to note that a drug that contains amphetamine only or metabolizes to amphetamine only will not result in a mass spectrometry positive result for methamphetamine. The only way to have methamphetamine in the urine is to consume a drug containing methamphetamine or one that metabolizes to methamphetamine. Methamphetamine will then metabolize to amphetamine. The following list is comprised of drugs that would be considered as true positives for amphetamine and methamphetamine.

Drugs that contain amphetamine:
  • Adderall
  • Benzedrine
  • Biphetamine
  • Dexedrine
  • Durophet
  • Procentra
  • Zenzedi
Drugs that contain methamphetamine:
  • Desoxyn (d-methamphetamine)
  • Vick's inhaler (l-methamphetamine)
  • Illicit methamphetamine
Drugs that metabolize to amphetamine:
  • Clobenzorex
  • Ethylamphetamine
  • Captagon (Fenethylline)
  • Tegisec (Fenproporex)
  • Pondinil (Mefenorex)
  • Prenylamine
  • Vyvanse
Drugs that metabolize to methamphetamine and amphetamine:
  • Didrex (Benzphetamine)
  • Dimethylamphetamine
  • Femprofazone
  • Fencamine
  • Furfenorex
  • Deprenyl (Selegiline)

I hope this helps in understanding false positives from a screening test (specifically an amphetamines urine immunoassay test) and the importance of a confirmatory test. I receive the question on a regular basis, so I thought I would address it here. And as I always say, when it doubt, confirm.


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