MTLAWS Drug Testing Screening Laboratory Practice Test

Immunoassays detect drugs by using antibodies that bind to the drug or a metabolite and produce a signal if binding occurs. This makes them very sensitive and fast, which is why they’re great for screening. But antibody binding isn’t perfectly specific; substances with similar structures can cross-react with the antibody and create a positive signal even if the target drug isn’t present, or if its level isn’t enough for a definitive identification. Because of this potential cross-reactivity and limited specificity, the result indicates a probable presence rather than a confirmed identification. That’s why positive results from immunoassays are followed by a confirmatory test using more selective methods like GC-MS or LC-MS/MS to identify the exact compound and quantify it. In short, the need to resolve possible false positives from cross-reactivity makes immunoassays presumptive tests.