Interpretation of analytical results is a complex operation with regard to any drug-testing result but is more complex with the amphetamines than it is for most drugs. Incorporated in the previous discussions of each of the drugs are details regarding interpretation of results. Some important considerations that must be remembered when interpreting the results of drug testing results that apply to all drugs, including amphetamines, is that most of the time the determination is made whether or not the results are consistent with use of the drug. It is far more difficult to demonstrate the unequivocal use of the drug.
It is important to remember several facts associated with metabolism and analysis that are critical to proper interpretation. These facts also allow interpretation that would otherwise be equivocal. These include the fact Mat the enantiomers do not change their orientation during any of the processes in the body (absorption, metabolism, distribution, and excretion). Therefore, if one of the enantiomers is not present in a drug, it cannot be excreted in urine. Methamphetamine is metabolized to amphetamine, but the reverse reaction is not seen in the body. As a result, administration of amphetamine does not lead to the excretion of methamphetamine. The fact that methamphetamine is excreted intact and is metabolized to amphetamine, which is also excreted, is an important metabolic fact. It not only tells us that amphetamine is an expected metabolite, but studies of the metabolism can also help understand what is expected. In this case, what is expected is more than the simple fact that the administration of methamphetamine will result in the excretion of methamphetamine and amphetamine. There are also proportions of these 2 drugs that are important to know and understand. A sample that contains amphetamine at a concentration greater than methamphetamine is not the result of administration of methamphetamine. Even if the proportion of a positive amphetamine sample has equal concentrations of amphetamine and methamphetamine, it is inconsistent with methamphetamine use. It could be the result of the use of a precursor compound such as benzphetamine or mixed use of amphetamine and methamphetamine at different times or different proportions.
For example, it is easy to exclude the use of a Vicks nasal inhaler when the methamphetamine in the sample is the d-enantiomer. The presence of ~enantiomer only unequivocally excludes the Vicks Whaler as the source. Likewise the presence of d-enantiomer only unequivocally excludes the use of famprofazone as the source of the methamphetamine. The presence of l-methamphetamine, however, does not prove the use of Methamphetamine nasal inhalers, the use of deprenyl also results in the presence of l-enantiomers only in a sample. Likewise, the presence of both d- and Methamphetamine does not demonstrate the use of famprofazone. Other sources of mixed enantiomer methamphetamine could also be the reason. In the case of the precursor compounds, the presence of a unique compound, either the parent drug itself or a unique metabolize, can clearly demonstrate the involvement of the drug. It does not by itself, however, demonstrate that all of the methamphetamine and/or amphetamine are the result of using that drug. To help make that determination, such things like proportions of the compounds compared to known proportions from controlled studies is an important consideration. Also important are the overall concentrations of the drugs. Although using urine drug concentrations in conjunction with the results from known administration is helpful, exclusion of what could not be involved is the strongest determination in demonstrating involvement of a specific drug. Comparison of proportions and concentrations are also very useful. The more analyte concentrations known regarding a sample, the more confident the determination that can be made.