Simply stated, screening and confirmation testing are performed using different testing methodologies that have different specificity and sensitivity. 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. The combined cross-reactivity of compounds in the drug class may elicit a positive response, even though an individual metabolite may be present below the initial test cutoff. When performing confirmation testing by GC/MS or LC/MS/MS, one or more specific metabolites can be identified, quantified and reported using the applicable confirmation cutoff for a positive test result.
