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An assay is an analytic procedure for qualitatively assessing or quantitatively measuring the presence or amount or the functional activity of a target entity. This can be a drug or biochemical substance or a cell in an organism or organic sample.
The measured entity is generally called the analyte, or the measurand or the target of the assay. The assay usually aims to measure an intensive property of the analyte and express it in the relevant measurement unit (e.g. molarity, density, functional activity in enzyme international units, degree of some effect in comparison to a standard, etc.).
If the assay involves addition of exogenous reactants (the reagents), their quantities are kept fixed (or in excess) so that the quantity (and quality) of the target is the only limiting factor for the reaction/assay process, and the difference in the assay outcome is used to deduce the unknown quality or quantity of the target in question. Some assays (e.g., biochemical assays) may be similar to or have overlap with chemical analysis and titration.