A comparison of enzyme immunoassay and gas chromatography/mass spectrometry in forensic toxicology

A series of 137 urine samples were analyzed for drugs of abuse by enzyme immunoassay (EMIT) and by gas chromatography/mass spectrometry (GC/MS). Agreement between these methods was excellent and ranged from 93.4% for benzodiazepines to 98.5% for propoxyphene. EMIT false negative were traced to the presence of elevated endogenous lysozyme or other interfering materials. In the case of moderate amounts of lysozyme the use of a blank would lead to correct results. Disagreement in the identification of nine benzodiazepine samples was found to be due to a low recovery of benzodiazepine metabolites from urine. Recovery could be improved by incubation of the urine sample with the enzyme beta-glucuronidase.