UDT is performed via a two-step process: an initial drug screen followed by confirmation or definitive testing for the drugs/drug classes. Most immunoassay screens detect the presence of a drug class (e.g. opioids) and not the individual drug (e.g. hydromorphone), so further testing might be necessary. Due to these limitations, more specific, definitive testing such as liquid chromatography tandem mass spectrometry (LC-MS/MS) should be used also. Due to mass spectrometry’s superior sensitivity and specificity, many laboratorians have advocated it as the first line testing methodology, particularly for frequently prescribed drugs such as opioids and benzodiazepines. Quantitative mass spectrometry also can provide information on the presence of particular metabolites, assisting in detecting urine adulteration or determining the parent compound(s) ingested.