Immunoassay Screening

The primary screening methods for detecting amphetamines are immunoassays. The structural similarity to amphetamine or methamphetamine of the compounds (shown in Fig. 1) makes it difficult to produce antibodies specific for amphetamine, methamphetamine, or both. Based on cross-reactivity studies, most amphetamine antibodies appear to be directed toward the amino group. Both monoclonal and polyclonal antibodies have been developed and used in amphetamine immunoassays. Monoclonal antibodies result in more defined specificities but not necessarily more selectivity. In general, amphetamine assays can be classified into three general types based on antibody specificity (2). One group consist of those assays highly selective for either amphetamine (and its designer counterpart MDA) or methamphetamine (and its designer drug counterpart MDMA) but not both sets simultaneously. A second group of immunoassays are those that are able to detect both amphetamine and metham-phetamine to varying extent but that also exhibit higher levels of cross-reactivity to the hydroxy amine compounds found in many OTC drugs. The third group of immunoassays consist of dual assays for amphetamine and methamphetamine with low levels of cross-reactivity to OTC drugs.

Quite often antibody specificity is influenced by the intended use of the testing. The term amphetamines is typically used to denote immunoassay testing specific for the two stimulants amphetamine and methamphetamine. Government-mandated and workplace drug testing typically specifies testing for amphetamine and metham-phetamine only. Laboratories performing workplace drug testing desire immunoassays specific for only those compounds specified in appropriate legislation or contracts. On the other hand, laboratories affiliated with an emergency department (ED) desire immunoassays directed toward the broad spectrum of sympathomimetic amines. This desire is articulated as a recommendation in the National Academy of Clinical Biochemistry's (NACB) Laboratory Medicine Practice Guidelines (LMPG) that the optimum immunoassays for amphetamines testing in ED patients are those directed toward phenylethyl amines as a class (3). These guidelines also recommend that the name of the test should be changed from "amphetamines" to sympathomimetic amines" or "stimulant amines."

Federally mandated testing currently includes a screening cutoff of 1000ng/mL for amphetamines, but a proposal by the Substance Abuse and Mental Health Services Administration (SAMHSA) to lower these levels to 500 ng/mL (and add MDMA) for screening is in the final stages of the regulatory process (4). The proposed changes are estimated to identify 5-24% more urine specimens containing amphetamines (4).

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