Samples that screen positive (those that are above the cutoff) are usually considered to be 'presumptively positive'. Many labs will then reflex these positive samples for confirmatory testing. Confirmatory testing uses more specific methodologies. Gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) are employed by the vast majority of clinical laboratories to confirm the presence of specific drugs and/or metabolites.
