GC-MS separates a specimen’s components by gas chromatography, then identifies each by its unique mass-spectral fingerprint — effectively eliminating the false positives that immunoassay cross-reactivity can produce. Only specimens that screen presumptively positive move to confirmation, and only results that exceed both the screening and the lower confirmatory cutoff levels are reported positive to the MRO. Liquid chromatography–tandem mass spectrometry (LC-MS/MS) applies the same principle to compounds that don’t vaporize well and is now equally common in certified labs. Together with chain-of-custody documentation, confirmation testing is what makes a workplace drug test result legally defensible.