Screening and confirmation testing use different methodologies. These methods require different cutoff levels. Immunoassay (screening) tests typically use higher cutoff levels than more sensitive GC-MS or LC-MS/MS confirmation tests. Screening tests detect a broader group of parent compounds, metabolites, and structurally similar substances. This broader detection range requires higher cutoff levels.
When a screening test detects a drug above its cutoff level, it flags the specimen as presumptive positive. The lab then sends the specimen for confirmation testing using GC-MS or LC-MS/MS. Confirmation testing identifies the specific drug or metabolite present. These individual compounds often appear in lower concentrations than the total response seen in an immunoassay. Therefore, GC-MS or LC-MS/MS tests use lower cutoff levels to detect them accurately.
Note: The term “false positive” usually refers to a non-negative result from a screening test that was not sent for confirmation. Screening tests “cast a wide net” to detect a broad range of substances. Some non-negative screening results turn out negative after confirmation with GC-MS.
