Enzyme multiplied immunoassay technique (EMIT) is a common method for qualitative and quantitative determination of drugs and certain proteins in serum and urine. First introduced by Syva Company in 1973, it is the first "homogeneous immunoassay" to be widely used commercially.
Homogeneous immunoassays can be carried out using exceptionally simple and rapid mix and read protocols. The most widely used applications for EMIT are for therapeutic drug monitoring (serum) and as a primary screen for abused drugs and their metabolites in (urine). The original US patents covering major aspects of the method, 3,817,837 and 3,875,011, have expired although US patent 6,455,288 that covers a major improvement is still valid. While still sold by Siemens Healthcare under its original trade name, EMIT, assay kits with different names that employ the same technology are supplied by other companies. Determination of drug levels in serum is particularly important when the difference in the concentrations needed to produce a therapeutic effect and adverse side reactions is small, the therapeutic window. EMIT therapeutic drug monitoring tests provide accurate information about the concentration of such drugs such as digoxin and other cardioactive drugs, theophylline, anti-epileptic drugs, and antibiotics. EMIT urine assays for abused drugs such as cannabinoids, morphine, and amphetamine are designed to detect the drug itself or a metabolite of the drug present in a concentration above a pre-specified minimum detection limit. The assays are highly accurate but, like all drug abuse assays, they are susceptible to attempts by the patient to circumvent the test by adulteration of the sample. Because of the social and legal consequences, a positive test result should be confirmed by an alternative method, usually mass spectroscopy.
Like all immunoassays, EMIT uses antibodies that are specifically designed to bind the molecule(s) of interest without binding to other substances in the sample. Its unique feature is the ability to detect this binding without resorting to a cumbersome separation of the bound component. This is accomplished by including in the mixture of antibodies and sample an enzyme that is attached to the drug. Antibodies that do not become bound to drug in the sample bind instead to this enzyme-drug "conjugate". The conjugate is designed in such a way that when antibodies bind to its drug portion, the enzyme is deactivated. The more drug there is in the sample, the more of the antibody is bound to it and less is available to deactivate the conjugate. If an enzyme substrate is present that is converted to a colored or fluorescent product, the presence of drug will inhibit the formation of the detectible product to a degree related to the concentration of the drug.
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