The Analytical Goal of a Drug Screening Assay

The goal of a drugs-of-abuse screening immunoassay is simpler than that of a clinical assay, which must accurately quantify normal serum components or abnormal markers over a range of concentrations. In forensic drug testing, there is usually just one standard reference, which contains the cut-off concentration of drug. Samples containing less than the cut-off drug concentration are considered negative, with no further testing required. Certain issues, such as cross-reactivity with related drugs or metabolites, which would produce unacceptable error in diagnostic clinical assays, are tolerable for workplace or other forensic screening because a second and more specific confirmatory test will be performed on the sample.

In an immunoassay, the essential aspects of sensitivity, precision, and accuracy are all hinged on the absence or control of sample-matrix effects. Sample-matrix effects are related to (1) the nature of the antibody, (2) the precision of the signal measurement, and (3) the level of nonspecific binding. Because hair-testing laboratories often purchase well-characterized immuno-assay kits from vendors, the selection of the antibody is usually not a factor. The job of the laboratory is to optimize the composition and amount of sample so that interference resulting from matrix and sample variability is minimized and accounted for. It is also important that the cut-off concentration be in the optimal response region of the assay. It should be noted that current US Food and Drug Administration (FDA) regulations require that testing intended for use outside of a medical setting (e.g., in the home, sports, or workplace) must use assays that have been cleared, approved, or otherwise recognized by the FDA as accurate and reliable for the specimen being tested (21 CFR 809; 21 CFR 864.326).

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