Evaluation of the 20% D-methamphetamine requirement for determining illicit use of methamphetamine in urine

In urine drug testing, enantiomer analysis is used to determine whether a positive methamphetamine result could be due to use of an over-the-counter (OTC) nasal inhaler containing L-methamphetamine. D-methamphetamine at more than 20% of the total is considered indicative of a source other than an OTC product. This interpretation is based on a 1991 Department of Health and Human Services (HHS) Technical Advisory. We performed studies to verify the methamphetamine enantiomer content of current OTC nasal inhalers and to evaluate current laboratory testing capabilities. This study demonstrated that OTC inhalers contain less than 1% D-methamphetamine. A proficiency testing (PT) set for HHS-certified laboratories performing methamphetamine enantiomer testing found D-methamphetamine percentages that were consistently 1 to 3% higher than theoretical due to optical impurity of the derivatizing reagent N-trifluoroacetyl-L-prolyl chloride (L-TPC). The PT results also demonstrate that laboratories can accurately determine 20% D-methamphetamine in samples with total methamphetamine concentrations down to 250 ng/mL. Based on these studies, the guideline of >20% D-methamphetamine is appropriate for interpreting results obtained using current laboratory methods.