Why are confirmatory tests such as GC-MS or LC-MS/MS required after a positive immunoassay result?

Why are confirmatory tests such as GC-MS or LC-MS/MS required after a positive immunoassay result?

• A. Because immunoassays are not sensitive.
• B. Because confirmatory tests are cheaper.
• C. Because GC-MS is faster than immunoassay.
• D. Because immunoassays are presumptive and may have cross-reactivity; confirmatory testing provides specific, quantitative identification of the drug and metabolite.

Answer: (D)

Immunoassays are screening tools that rely on antibodies to detect a drug or its class, but those antibodies can bind to structurally similar substances as well. That cross-reactivity means a positive result can occur even if the exact drug isn’t present, making the result presumptive rather than definitive. Confirmatory testing with GC-MS or LC-MS/MS goes beyond screening by separating the sample’s components and measuring a specific mass spectral fingerprint for the drug and its metabolites. This provides both positive identification and precise quantitation, distinguishing the target substance from other compounds that might have triggered the initial signal. That specificity and the quantitative data are why confirmatory tests are required after a positive immunoassay.

Immunoassays are screening tools that rely on antibodies to detect a drug or its class, but those antibodies can bind to structurally similar substances as well. That cross-reactivity means a positive result can occur even if the exact drug isn’t present, making the result presumptive rather than definitive. Confirmatory testing with GC-MS or LC-MS/MS goes beyond screening by separating the sample’s components and measuring a specific mass spectral fingerprint for the drug and its metabolites. This provides both positive identification and precise quantitation, distinguishing the target substance from other compounds that might have triggered the initial signal. That specificity and the quantitative data are why confirmatory tests are required after a positive immunoassay.