Diagnostic Use
Plasma testing for amphetamines is a laborious process that requires specialist technician time. In the vast majority of cases, urine amphetamine testing is preferred. Plasma testing may be useful when the patient cannot provide a urine sample for testing.
This plasma method detects Amphetamine, Methamphetamine and MDMA. This plasma amphetamine assay is not intended to detect novel psychoactive substances (e.g. cathinone) or other amphetamines such as MDEA, MDA, PMA, ephedrine and phentermine.
This test differs from the urine amphetamine test (see amphetamine urine offered as part of a urine drug screen, initially by immunoassay and confirmed, if necessary, by mass spectrometry. The urine immunoassay detects more amphetamine-type substances but does not differentiate between them.
Interpretation
Methamphetamine:
- Metabolised to amphetamine, thus when methamphetamine is consumed, amphetamine is expected to be detected also (though often at a much lower concentration).
- There are two stereoisomers of methamphetamine that cannot be distinguished by this laboratory method:
- D-methamphetamine – Generally observed in illicit drug use. Short plasma half-life (6-15 hours). May be undetectable after 24-48 hours.
- Reported correlation between plasma levels and symptoms:
- 50 – 2600 ug/L – reported in impaired drivers [1].
- > 100 ug/L – Rapid or confused speech, dilated pupils, agitation, paranoia and rapid pulse [3].
- 300 ug/L – Violent behaviours associated with arrest [3]
- L-methamphetamine – This isomer has a weaker central stimulant action and may be found in some nasal decongestants or metabolites of selegiline. Half-life is typically around 13-15 hours in plasma.
Amphetamine:
- Peak levels occur within minutes of IV administration, around 30mins of intramuscular, topic or nasal route and within 2-3 hours of oral ingestion.
- May be used as a treatment for ADHD and narcolepsy.
- Half-life is typically between 7-34 hours. 30-40% of amphetamine is excreted unmodified by the kidneys, with the clearance rate dependent on urine pH.
- In 48 adults with impaired driving amphetamine, concentrations ranged from 30 – 2300 ug/L.
- Time to positive plasma results varies between 1-24h (positive > 2.5 ug/L).
Methylenedioxymethamphetamine (MDMA; Ecstasy):
- The half-life is between 5-9 hours [1].
- Troublesome after-effects may occur for one to two days after ingestion (5 half-lives = 40 hours).
- Typical doses used for recreational purposes varies between 50 – 150 mg. Recreational use of MDMA typically produces blood levels of > 100 ug/L [5]
Limitations / Interference
This plasma amphetamine assay is not intended to detect novel psychoactive substances (e.g. cathinone) or other amphetamines such as MDEA, MDA, PMA, ephedrine and phentermine.
This mass spectrometry method cannot distinguish between L and D forms of methamphetamine.
Uncertainty of Measurement
Amphetamine: 16%
Methamphetamine: 16%