Urine drug testing companies have their own sophisticated laboratories which perform UDT with greater accuracy than that provided by in-office immunoassay screening. When an immunoassay test is inconsistent (i.e. indicating the presence of a prescription or illicit drug that should not be in the sample, or, conversely, not containing evidence of a medication that should be in the sample), more expensive and accurate confirmation testing should be conducted. According to a large retrospective study, between 3 and 5% of screening results should be sent for confirmation testing.
From an ethical perspective, confirmation testing should be performed on all inconsistent immunoassays, as the repercussions of a false-positive screen can be dramatic. Confirmation testing was typically conducted through gas chromatography/mass spectrometry (GC/MS), although recent advances in technologies such as liquid chromatography-tandem mass spectrometry (LC-MS/MS) are beginning to bring about a paradigmatic revision in laboratory practices. Importantly, the validity for extreme accuracy of both of these types of testing has been established.