A methamphetamine-to-amphetamine ratio can contribute to a pharmacokinetic interpretation, but a single ratio does not reliably determine an individual’s exact time of use. The parent drug and metabolite have different concentration-time profiles, and repeated doses can obscure their relationship.
Why the ratio changes
Logan’s 2002 review describes metabolism of methamphetamine to amphetamine. In the controlled oral studies summarized there, the methamphetamine peak preceded the peak of the amphetamine metabolite. Consequently, a relatively high parent-to-metabolite ratio could be compatible with more recent exposure under those experimental conditions.
The review explicitly cautions that multiple doses confound that interpretation. A later dose adds parent drug to a specimen that may already contain metabolite from earlier administrations. The resulting ratio need not resemble the curve after a single isolated dose.
The specimen must match the reference data
The review discusses plasma, serum, urine and saliva findings from different studies. A ratio measured in one compartment should not be assigned a timing rule developed in another without supporting validation. Urinary excretion and a circulating concentration are different measurements.
Reporting limits also matter. If amphetamine is below the method’s quantification limit, dividing the methamphetamine value by an assumed zero does not produce a meaningful quantitative clock. Units and the laboratory’s handling of low values must be explicit.
Elimination is variable
The historical review reports broad half-life ranges and notes the importance of urinary pH to methamphetamine disposition. Its controlled-dose summaries cannot supply an exact elimination rate for an unknown individual with uncertain dosing history.
Other potential sources of amphetamine or methamphetamine further complicate interpretation. A ratio should therefore be examined together with medication history, analyte confirmation and, where relevant, stereochemistry.
How to frame a forensic conclusion
Ask whether the proposed estimate comes from a validated model for the same specimen, population, route and dosing pattern. Distinguish a pattern compatible with recent use from a proven administration time. If several histories can generate the finding, disclose that ambiguity.
Finally, a time-of-use inference is not an impairment measurement. Even an improved estimate of exposure timing would still require a separate assessment of functional performance at the event.
Scientific source
Logan BK. Methamphetamine—Effects on Human Performance and Behavior. Forensic Science Review. 2002;14(1–2):133–151. This article analyzes the supplied historical review; studies discussed within that review are secondary-source evidence here.
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