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98) The enzyme-multiplied immunoassay technique (EMIT) is a homogeneous immunoassay that is used to measure low molecular weight molecules. In an EMIT assay, which one of the following best explains how the enzyme-labeled antigen that is bound to the antibody is distinguished from the non–antibody-bound, free enzyme-labeled antigen?

The enzyme-labeled antigen activity is inhibited by changing the pH of the reaction. The antibody-bound enzyme-labeled antigen complex is not as stable as the free, non–antibody-bound enzyme-labeled antigen complex, and the enzyme activity is measured by a kinetic rate reaction. Antibody binding to the enzyme-labeled antigen enhances the enzyme activity. Antibody binding to the enzyme-labeled antigen sterically inhibits the enzyme activity. The free, non–antibody-bound enzyme-labeled antigen is fluorescent.

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• The EMIT assay is an automated homogeneous enzyme immunoassay that is used primarily for measuring drugs.

• An enzyme (usually glucose-6-phosphate dehydrogenase) is labeled with a drug (i.e., enzyme-bound antigen) and competes with the free drug in the sample (i.e., the free antigen) for binding sites on a drug-specific antibody. After the reaction is complete, enzyme substrate is added and the amount of enzyme product produced is measured spectrophotometrically.

• The enzyme-labeled drug that is bound to the antibody is sterically inhibited from reacting with the enzyme substrate. Only the free, non–antibody-bound enzyme-labeled drug reacts with the enzyme substrate.

• The concentration of the drug in the patient sample is directly related to the amount of enzyme product produced.

• The EMIT assay can be used with any spectrophotometer and is the most common procedure used for quantifying serum drug levels.

Mendru DR, Chou PP, Soldin SJ: An improved application of the enzyme multiplied immunoassay technique for caffeine, amikacin, and methotrexate assays on the Dade-Behring dimension RxL Max Clinical Chemistry System. Ther Drug Monit 2007;5:632-637.

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Clinical Pathology: General Principles, Lab Management, Clinical Chemistry

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