Interpretation of amfetamine positive results
This particular drug-testing result has caused, and continues to cause, confusion among the MRO community. Some of this stems from the reporting rule for methamfetamine. This rule requires that for methamfetamine to be reported as positive there must be at least 200 ng/mL of amfetamine present, in addition to at least 500 ng/mL of methamfetamine. If both amines are present in concentrations greater than 500 ng/mL by GC-MS, both are reported as positive and there is no confusion. Difficulty arises when the amfetamine concentration determined by the laboratory is between 200 and 500 ng/mL (i.e. less than the amfetamine confirmation cut off), but the methamfetamine concentration is above 500 ng/mL. In this situation, the MRO receives only a methamfetamine-positive result, rather than one that indicates that both methamfetamine and amfetamine are positive. Once the MRO understands that the laboratory must comply with this reporting rule, the confusion disappears.
The second area of confusion is in the differentiation of d-and l-methamfetamine. l-Methamfetamine is found in the USA in Vicks Inhaler (an over-the-counter decongestant product) and, although its use can result in the detection of methamfetamine and amfetamine in urine, the concentrations are generally low (Fitzgerald et al. 1988). Moreover, given the low cross-reactivities of immunoassays to the l isomers of methamfetamine and amfetamine, a positive result is unlikely following normal use. However, a methamfetamine user may still claim that the positive arises because of his or her use of Vicks Inhaler and, in such cases, the MRO can request a GC-MS separation of the isomers. These are usually qualitative analyses and reported as x% of the d-isomer and y% of the l isomer. Where the l-isomer is greater than 80%, the MRO reports the result as negative. Although some drugs can metabolise to methamfetamine and amfetamine (e.g. benzphetamine and selegeline), these are available only on prescription in the USA and the interpretation of positive amfetamine results is relatively straightforward. In other countries, this may not be the case. Cody (2002) recently reviewed the issue of precursor medications.
