In toxicology testing, how do screening immunoassays differ from confirmatory methods like GC-MS or LC-MS/MS?

• A. Immunoassays are rapid and broad but may have cross-reactivity and false positives; GC-MS/LC-MS/MS provide definitive identification and high specificity.
• B. Immunoassays are slower but more specific than GC-MS.
• C. GC-MS is less sensitive than immunoassays.
• D. Immunoassays can identify all compounds definitively.

Answer: (A)

Screening immunoassays are designed for rapid, broad detection with high sensitivity across many substances, making them useful for initial triage. Their results can be affected by cross-reactivity, where antibodies bind to structurally similar compounds or metabolites, which can produce false positives. Because of this limited specificity, a positive immunoassay is not considered definitive.

Confirmatory methods like GC-MS or LC-MS/MS provide definitive identification thanks to chromatographic separation and mass spectral analysis. They reveal the exact compound by its retention time and distinctive fragmentation pattern, offering high specificity and accurate, often quantitative, results. These methods are more time-consuming and resource-intensive, which is why they are used to confirm positives from screening tests (and to precisely identify and quantify substances).