An assay is an analytic test to detect the presence, measure the concentration, and determine the biological activity of molecule, ions, or cells. Assays are based on measurable parameters that enable the evaluation of differences between samples and controls. Sensitivity, the ability to detect small amounts of a substance, and specificity, the ability to detect only the analyte, are essential features of an assay.

See also Bioassay, Biochemical, Biohazards, Biokinetic Model and Bioaccumulation Factor.

A bioassay is a medical test to measure, and quantify radionuclides in the body by direct (in vivo) or indirect (in vitro) analysis of tissues or excretions from the body.

See also Radioimmunoassay.

A radioassay is an in vitro test to detect hormones, drugs and medicines in smallest concentrations.

(RIA) A radioimmunoassay is an in vitro assay test in which very small quantities of hormones, drugs and other substances in blood, urine, etc. can be measured by using specific antibodies or antigens which have been labeled with radioactive tracers. Since the patient does not receive the radioactive material, there is no patient radiation exposure.
Radioimmunoassays are usually replaced by fluoroimmunoassay and enzymimmunoassay to avoid the use of radioactive materials.

See also Bioassay, Fluorescent Immunoassay, Protein Binding Assay, Radioassay, and Berson Salomon.

(EIA) Enzyme immunoassays have been developed as an alternative to radioimmunoassays. These methods use an enzyme to label either the antibody or antigen. The sensitivity of an enzyme immunoassay is similar to that for a radioimmunoassay, without the danger of radiation. One of the most widely used EIA for detection of infectious diseases is the ELISA (Enzyme Linked lmmuno Sorbent Assay) method.

See also Fluorescent Immunoassay.