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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Immunoassay Techniques, continued

In comparison with other EIAs, ELISA is more sensitive and more specific. However, ELISA is not easy to automate because the incubation time is longer. EMIT and CEDIA are preferred over ELISA for routine drug testing, but ELISA is the preferred choice for more specific analysis.
Radio immunoassay (RIA) was one of the earliest immunoassays developed for drug testing but is no longer widely used because of the potential dangers and costs involved with radioactive isotopes.
Quantitative results are not determined in EIAs. Rather, qualitative results of classes of drugs or metabolites are reported in terms of being below or above the cutoff. A cutoff is a concentration above which an analyte is reported as present or positive and below which it is reported as absent or negative.
Not all drugs can bind antibodies and thus be detected by immunoassays. Some examples are ethanol/other alcohols and ethyl sulfate. These compounds are too small to analyze by EIA.