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An assay is an investigative (analytic) procedure in laboratory medicine, pharmacology, environmental biology and molecular biology for qualitatively assessing or quantitatively measuring the presence, amount, or functional activity of a target entity (the analyte). The analyte can be a drug, biochemical substance, or cell in an organism or organic sample. The measured entity is often called the analyte, the measurand, or the target of the assay. An assay usually aims to measure an analyte’s intensive property and express it in the relevant measurement unit (e.g. molarity, density, functional activity in enzyme international units, degree of effect in comparison to a standard, etc.).