Homogeneous enzyme immunoassay in principle and practice

Abstract

Of the assays developed to measure drugs and hormones, homogeneous enzyme immunoassay has proved to be unique in that it is not only quick and easy, but also has high specificity and is thus effective in enhancing patient care. The technique utilizes an enzyme instead of a radioactive label and unlike radioimmunoassay, requires no separation of bound and free species. Specific antibodies can bind either the enzyme labelled conjugate or the unlabelled form. Binding of the enzyme conjugate causes either a diminution or increase in enzyme activity and this change depends on the amount of unlabelled drug or hormone from the patient sample which also competes for antibody sites. Observed enzyme activity can thus be correlated with drug or hormone level in the patient sample. An important feature in the development of such assays is the specialized chemistry needed to covalently link the drug or hormone or one of their congeners to the enzyme. Procedures are now available (EMIT ® ) for the assay of antiepileptic, antiasthmatic and cardioactive drugs as well as T 4 and these can be performed using appropriate manual spectrophotometric equipment and certain automatic enzyme analysers.