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Immunoassay Techniques

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p363-364

2026-08-12

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Immunoassay Techniques

Immunoassay techniques take advantage of the reaction that takes place between antigens and antibodies in human blood.“Serology.” An antigen is a substance that is resident on red blood cells and in the blood plasma itself. If a substance such as a virus or bacterium enters the body, it acts like a foreign antigen and causes the immune system to produce antibodies that can attack and immobilize the antigen before it can cause harm. The antibodies produced are specific for the type of antigen that is introduced. In forensic toxicology, this antigen/antibody reaction is exploited to detect very small quantities of drugs. A major advantage of immunoassay tests for drug screening is that they do not require prior extraction of the drugs from the urine. One of the most popular immunoassay tests is called the Enzyme Multiplied Immunoassay Test (EMIT). A rabbit is injected with a form of the drug for which detection is sought, for example, methamphetamine. The rabbit’s immune system pro duces an antibody to the methamphetamine. The rabbit’s serum is removed. A known amount of the rabbit serum is added to the urine of the human subject being tested for drugs. If there is any methamphetamine in that person’s urine, it will immediately bind to the antibodies. Next, a known amount of methamphetamine that has been labeled with an enzyme is added to the urine. If there are any antibodies from the rabbit that did not react with the methamphetamine in the subject’s urine, they will now react with the enzyme-labeled methamphetamine, thus decreasing its concentration.

The amount of the remaining enzyme-labeled methamphetamine is now measured. This will correlate to the concentration of the antibodies that reacted and thus to the concentration of the antibodies that reacted with the methamphetamine that was in the urine. From this, the concentration of the methamphetamine can be determined. Another related immunoassay technique is called Radioimmunoassay (RIA). RIA works in a similar manner to EMIT but the antigens are radioactively labeled. The major problem with RIA and EMIT is that the antibodies produced in the rabbit from injecting a drug are not generally specific for that drug. For example, antibodies produced as a result of the injection of methamphetamine into a rabbit may also bind to pseudoephedrine, a proprietary antihistamine sold in drug stores for allergy relief. Relying solely on an immunoassay test for identification of methamphetamine would mean that a false positive result would occur. Thus, all immunoassay tests as well as chromatographic screening tests must be confirmed with a suitable second test.