Homogeneous Enzyme Immunoassay (EMIT)

Forensic medicine and hospital laboratories utilize several different types of biochemical assays (tests) for drug detection in body fluids and tissues, including liquid chromatography-mass spectrometry, high-performance liquid chromatography, and immunochemical techniques, among others. Immunochemical techniques identify chemicals in urine, blood plasma, or tissues through the reactions between body antigens and antibodies in the presence of a foreign protein or chemical.

Cells present fragments of intracellular proteins to T cell lymphocytes (a type of white blood cell) that scan tissues in search of foreign pathogens such as viruses, bacteria, or toxins. When a foreign protein (or toxin) is detected by T cells, an immune response is triggered and antibodies are produced by B-lymphocytes and sent to bind to these alien proteins. Antibodies are proteins that enhance antigen recognition by other cells of the immune system. Enzyme-Multiplied Immunoassay Technique (EMIT) was introduced in 1972 for the rapid detection of hormones and drug metabolites in human fluids.

Antibodies against abused drugs are synthesized in laboratories to make them recognizable as foreign entities by a B-lymphocyte (type of white blood cell). Drug molecules are attached to a high molecular weight protein to form an antigen conjugate. These antigen conjugates are then injected into a host animal, whose immune system will produce drug-specific antibodies. The resulting antibodies may be of two types, either monoclonal or polyclonal anti-bodies. Monoclonal antibodies are families of identical proteins that only bind to a specific site of an antigen molecule, whereas polyclonal antibodies are not identical and bind to more than one antigen site. Once the desired blood levels of antibody are obtained, antibodies are recovered from the animal blood and purified.

EMIT detects even small quantities of drugs and drug metabolites (drug-derived molecules) in biological fluids, such as blood and urine. EMIT assays can be run in two different ways: Competitive EMIT and Non-competitive EMIT. Competitive EMIT contains drug antigens which will compete for the same antigen sites with the drug under investigation that is present in the body fluid sample. In a non-competitive EMIT assay, the drug under investigation reacts with a labeled antibody protein to form a colored substance. The following drugs are detected by EMIT: cocaine and metabolites, cannabinoides, opiates, amphetamines, phentanyl, methadone, barbiturates, benzodiazepines, phenylciclidine, and propoxyphene.