Screening and confirmation testing are performed using different methodologies that require different cutoff levels. The cutoff levels of an immunoassay (screen) are typically higher than those of a more sensitive GC-MS or LC-MS/MS confirmation test. The reason is that they test for a larger group of parent compounds, metabolites and other structurally similar compounds.
If a screening test detects a drug (above the screening cutoff level) the presumptive positive specimen is then sent for confirmation testing (GC-MS or LC-MS/MS ). Confirmation testing identifies the specific drug or metabolite. Often these individual compounds are present in concentrations much lower than the total immunoassay response, thus the need for lower cutoffs for GC-MS or LC-MS/MS testing.
Note: When you hear the term “false positive,” it generally refers to a test that was non-negative on an immunoassay test and not sent for confirmation testing. The nature of a screening test is to “cast a wide net.” Some non-negative screening results end up negative when sent for GC-MS.