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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, a biochemical substance, or a cell in an organism or organic sample. The measured entity is generally called the analyte, 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), then 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. But generally, assays involve biological material or phenomena which tend to be intrinsically more complex either in composition or in behavior or both. Thus reading of an assay may be quite noisy and may involve greater difficulties in interpretation than an accurate chemical titration. On the other hand, older generation qualitative assays, especially bioassays, may be much more gross and less quantitative (e.g., counting death or dysfunction of an organism or cells in a population, or some descriptive change in some body part of a group of animals).

Assays have become a routine part of modern medical, environmental, pharmaceutical, forensic and many other businesses at various scales from industrial to curbside or field level. Those assays that are very highly commercially demanded have been well investigated in research and development sectors of professional industries, undergone generations of development and sophistication, and in some cases are protected by the regulation of the use of Intellectual property such as patents granted for inventions or various regulatory incentives and exclusive rights. Such industrial scale assays as these are often done in well equipped laboratories and with automated organization of the procedure´from ordering an assay to pre-analytic sample processing (sample collection, necessary manipulations e.g. spinning for separation or other processes, aliquoting if necessary, storage, retrieval, pipetting/aspiration etc.). Analytes are generally tested in high throughput AutoAnalyzers, and the results are verified and automatically returned to ordering service providers and end users. These are made possible through use of advanced Laboratory informatics system that interfaces with multiple computer terminals with end users, central servers, the physical autoanalyser instruments, and other automata.

An assay (analysis) is never an isolated process and must be combined with pre- and post-analytic procedures. The information communication (e.g. request to perform an assay and further information processing) or specimen handling (e.g. collection, transport and processing) that are done until the beginning of an assay are the preanalytic steps. Similarly, after the assay, the result may be documented, verified and transmitted/communicated in steps that are called post-analytic steps. Like any multi-step information handling and transmission systems, variation and errors in the communicated final results of an assay involve corresponding parts in every such step; i.e. not only analytic variations and errors intrinsic to the assay itself but also variations and errors involved in preanalytic and post analytic steps. Since the assay itself (the analytic step) gets much attention, steps that get less attention by the chain of users, i.e. the preanalytic and the post analytic steps, are often less stringently regulated and generally more prone to errors – e.g. preanalytic steps in medical laboratory assays may contribute to 32–75% of all lab errors.

A cell-counting assay may determine the number of living cells, the number of dead cells, or the ratio of one cell type to another, such as numerating and typing red versus different types of white blood cells. This is measured by different physical methods (light transmission, electric current change). But other methods use biochemical probing cell structure or physiology (stains). Another application is to monitor cell culture (assays of cell proliferation or cytotoxicity). A cytotoxicity assay measures how toxic a chemical compound is to cells.