False Positives in Toxicology Testing

By Kevin Shanks, M.S., D-ABFT-FT

We talk about false positives in forensic toxicology a lot. Could a specific drug cause a false positive for another drug on a toxicology test?  

The answer is a bit complex, but it is both yes and no. 

First, we need to talk about the screening test. One of the most commonly used screening tests is immunoassay. Immunoassays 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.”  

Let’s look at an amphetamine and methamphetamine as an example for this false positive phenomena.  

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 (Gocovri) 
• Bupropion (Wellbutrin) 
• Chlorpromazine (Thorazine) 
• Desipramine (Norpramin) 
• Ephedrine (Ephedra) 
• Labetalol (Trandate) 
• Mexiletine (Mexitil) 
• Phentermine (Adipex) 
• Pseudoephedrine (Sudafed, Mucinex-D) 
• Trazodone (Desyrel) 

As you can see from the list, simple use of an over the counter nasal decongestant such as Mucinex-D or Sudafed (pseudoephedrine) could lead to a positive immunoassay screening test for amphetamines. Or the use of prescription medication Adipex (phentermine) or Wellbutrin (bupropion) could easily cause a positive immunoassay test for amphetamines. 

Second, we need to talk about confirmatory tests. While the screening test can be valuable for interpretation of 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.  

Going back to the amphetamines example, while the scope of a confirmation assay is highly dependent on the individual laboratory doing the analysis, the routine confirmatory amphetamines test typically only monitors amphetamine, MDMA, and methamphetamine. Some labs offer an expanded amphetamines panel and may include compounds such as ephedrine, MDA, MDEA, and pseudoephedrine or even other novel psychoactive substances such as the substituted cathinones (Dimethylpentylone, Eutylone, N-ethylpentylone, etc.). 

If a mass spectrometry-based confirmatory test is positive for amphetamine or methamphetamine, the individual being tested 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 a confirmed positive methamphetamine result 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. 

These are drugs that contain amphetamine or metabolize to amphetamine: 

• Adderall 
• Benzedrine 
• Dexedrine 
• Durophet 
• Procentra 
• Zenzedi 
• Ethylamphetamine 
• Captagon (Fenethylline) 
• Tegisec (Fenproporex) 
• Pondinil (Mefenorex) 
• Vyvanse (Lisdexamphetamine) 

These are drugs that contain methamphetamine or metabolize to methamphetamine: 

• Desoxyn (d-methamphetamine) 
• Vick’s vapo-inhaler (l-methamphetamine) 
• Illicit methamphetamine 
• Didrex (Benzphetamine) 
• Dimethylamphetamine 
• Gewodin (Femprofazone) 
• Altimina (Fencamine) 
• Deprenyl (Selegiline) 

At Axis Forensic Toxicology, we do not do preliminary screening by immunoassay testing. We have moved to more specific and selective initial screening using liquid chromatography with quadrupole time of flight mass spectrometry (LC-QToF-MS). All confirmatory testing is completed by either gas chromatography with mass spectrometry (GC-MS) or liquid chromatography with triple quadrupole mass spectrometry (LC-MS/MS). 

Axis’ Comprehensive Panel includes Analyte Assurance™ for novel and designer compounds. While Axis’ screening methodology dramatically reduces “false positive” results, the confirmation of these compounds via a second test remains important if the results are to be used in forensic findings.

If you have any questions or concerns about potential false positives in your forensic toxicology casework, please contact our toxicologist subject matter experts at [email protected]. 

To stay current with the scope of testing for all realms of toxicology offered by Axis Forensic Toxicology, please consult the online test catalog. 

References 

Guidelines for the Interpretation of Analytical Toxicology Results. Disposition of Toxic Drugs and Chemicals in Man. Twelfth Edition. Randall C. Baselt. Biomedical Publications. Pages xxx-xlii. (2020).  

Postmortem Forensic Toxicology. Principles of Forensic Toxicology. Fifth Edition. Barry Levine and Sarah Kerrigan. Springer. Pages 3-14. (2020). 

Forensic Drug Testing. Principles of Forensic Toxicology. Fifth Edition. Barry Levine and Sarah Kerrigan. Springer. Pages 45-64. (2020). 

Chromatography. Principles of Forensic Toxicology. Fifth Edition. Barry Levine and Sarah Kerrigan. Springer. Pages 135-162. (2020). 

Immunoassay. Principles of Forensic Toxicology. Fifth Edition. Barry Levine and Sarah Kerrigan. Springer. Pages 177-196. (2020). 

Mass Spectrometry. Principles of Forensic Toxicology. Fifth Edition. Barry Levine and Sarah Kerrigan. Springer. Pages 197-220. (2020).