Diagnosis:
Immunoassays for tier I drug screening
• Tricyclic antidepressants, barbiturates, most amphetamines, methamphetamine, cocaine, cannabinoids, phencyclidine, morphine, heroin, codeine, methadone, and ethyl alcohol are adequately detected by most automated immunoassays.
• Benzodiazepines are adequately detected by most automated immunoassays. Urine screens might be preceded by a preparation step of deglucuronidation.
• From a 2008 investigation using the College of American Pathologists Urine Drug Testing Surveys, it became clear that most laboratories correctly report that samples are positive for the following:
• Tricyclic antidepressants when challenged with urine samples that contained amitriptyline at a concentration of 1300 ng/mL, using a cutoff of 1000 ng/mL.
• Benzodiazepines when challenged with urine samples that contained lorazepam at a concentration of 750 ng/mL, using a cutoff of 300 ng/mL.
• Opiates when challenged with urine samples that contained hydromorphone at a concentration of 1000 ng/mL, using a cutoff of 300 ng/mL.
• Only 17% of laboratories correctly report that samples are positive for opiates when challenged with urine samples that contained oxycodone at a concentration of 7500 ng/mL, using a cutoff of 300 ng/mL. This percentage was even lower (i.e., 2.5%) when challenged with samples that contained oxycodone at a concentration of 1500 ng/mL. These failures result from differences in immunologic cross-reactivity for various opioids. A similar problem exists when testing for amphetamines.
• Immunoassays that are used to detect drugs of abuse rely on cross-reactivity with other compounds of the same class. This approach works better for some classes of drugs than for others.
