False Positive Urine Test for Methamphetamine?
Why urine screens produce false positives for methamphetamine, and how hair testing can set the record straight.
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False positives for methamphetamine on initial urine immunoassay screens are well documented. These quick screens are presumptive tests, not definitive ones. They use antibodies to detect chemical structures similar to methamphetamine, so certain prescription and over-the-counter medications can trigger a positive in someone who has taken no methamphetamine at all.
Medications that may trigger one
- some beta blockers, such as labetalol
- certain antidepressants, such as bupropion and trazodone
- decongestants, including pseudoephedrine and phenylephrine
- ranitidine, a heartburn medication now largely withdrawn
- selegiline, a Parkinson’s medication that metabolises into l-methamphetamine
This is a limitation of the screening method rather than a mistake by the person reading it, and the same principle applies to roadside saliva screening.
Why a screen is not the end of it
A positive urine screen is presumptive. Read alongside the person’s presentation, history and medication profile, it needs confirmation before any conclusion is drawn.
Testing a hair sample by GC–MS (gas chromatography–mass spectrometry) is one way to get that confirmation. Mass spectrometry identifies the exact compound present rather than reacting to a similar shape, so it distinguishes methamphetamine from the prescription substances that cross-react on a screen. It also covers a much longer window than urine — a single screen speaks to the past few days, while a hair sample covers months.
How the findings are interpreted
Methylamphetamine alone, no amphetamine detected. Most likely methylamphetamine use, though environmental contamination cannot be excluded in every case.
Amphetamine alone, no methylamphetamine detected. Most likely dexamphetamine, which may be lawfully prescribed or not. Dexamphetamine and amphetamine are two names for essentially the same substance.
Both present. Amphetamine is usually a by-product of methylamphetamine use. It is not always possible to tell whether some of the amphetamine also came from dexamphetamine.
For the longer treatment of this — how the metabolites work, and what genuinely confirms illicit use — see methylamphetamine and amphetamine in hair drug testing.
Expert interpretation matters
If you have been accused on the strength of a screening result, or you simply need to know what your report means, Dr Michael Robertson can review it and give an independent opinion — in a form you can put to a court, an employer or a treating doctor.
Request a consultation or written report, or call us on (03) 9380 5434.
