Specificity of the EMIT drug abuse urine assay methods

A investigation was conducted to determine the specificity of the EMIT DAU method of drugs of abuse analysis. Drug-free urine, from healthy volunteers, was individually spiked at 1000, 100, 10, and 1 microgram/mL concentrations with each of 162 different drug substances. These spiked samples were analyzed with the EMIT DAU assay for amphetamines, barbiturates, benzodiazepine metabolites, cocaine metabolites, methadone, opiates, and propoxyphene. Although several of the test methods yielded positive results at a concentration of 100 micrograms/mL, many drugs will probably not reach that concentration in the urine. The number of drugs giving a false positive at a concentration of 100 micrograms/mL was very low. The assay for cocaine metabolites gave no false positive results at any of the concentrations studied while the assay for methadone gave the largest number of false positive results. When interpreting the results of this investigation one must consider that in many cases drug metabolites will exist in the urine, salt forms of the drugs studied were used, and ionic strength and pH effects can interfere with the lysozyme enzyme system used in the EMIT DAU assays. In summary, the proper utilization of specificity information may assist the analyst in explaining unusual values obtained in the laboratory, particularly when the subject is concurrently using prescription or nonprescription medication.