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.
Gas chromatography/mass spectrometry (GC/MS) and liquid chromatography with tandem mass spectrometry (LC/MS/MS) are the methods Labcorp uses to confirm presumptive positive drug screens. These mass spectrometric methodologies provide identification of the molecule(s) based on characteristic fragmentation patterns at specific retention times. GC/MS and LC/MS/MS are hyphenated techniques, utilizing a gas or liquid chromatograph coupled to single- or multiple-stage mass spectrometers.
Labcorp performs initial drug screening using immunoassay. An immunoassay is a test that uses antibodies to detect the presence of drugs and other substances in urine. The initial screening process does not measure the specific amount of drug present in urine samples. It provides either a negative or a presumptive positive result, indicating the absence or possible presence of detectable drug relative to a ‘threshold’ or ‘cutoff’ concentration.