Which analytical method is typically used for confirmatory drug testing after an immunoassay screen?

Which analytical method is typically used for confirmatory drug testing after an immunoassay screen?

• A. High-performance liquid chromatography (HPLC)
• B. Gas chromatography–mass spectrometry/mass spectrometry (GC-MS/MS)
• C. Enzyme-linked immunosorbent assay (ELISA)
• D. Infrared spectroscopy

Answer: (B)

After a screening immunoassay flags a potential drug, confirmation needs a method that can definitively identify the exact substance and quantify it in the sample. GC-MS/MS fits this role because it combines a reliable separation (gas chromatography) with highly specific identification (tandem mass spectrometry). The MS/MS step looks at characteristic fragment ions from a known precursor, providing a unique fingerprint for the drug and its metabolites. This level of specificity greatly reduces the chance of false positives that can arise from cross-reactivity in immunoassays, and it allows accurate quantitation in complex biological matrices like urine or blood. That blend of separation, definitive identification, and quantitation is why GC-MS/MS is the go-to confirmatory method after an initial immunoassay screen.

ELISA is itself an immunoassay used for screening, not for confirmatory identification. Infrared spectroscopy generally lacks the sensitivity and specificity needed for confirmatory testing in biological samples. HPLC can separate components and, when coupled with an appropriate detector, may be useful, but GC-MS/MS provides a level of structural confirmation and widely validated performance that is standard for definitive drug identification.