Why are screening and confirmation cut-off levels different?
Simply stated, screening and confirmation testing are performed using different testing methodologies that precipitate different cut-off levels. The immunoassay tests used to perform initial drug screening are designed to detect a wide range of chemically similar compounds that react with the antibodies which are at the core of the chemistry making up the tests. In contrast, GC/MS confirmatory testing detects specific metabolites that provide identification and quantification of a specific drug.
