Difference Between Assay and Potency

The key difference between assay and potency is that an assay is the testing of a material to determine its ingredients and quality whereas potency is the amount of a drug required to get an effect at its maximum intensity. Usage of these two terms, assay and potency, are common in biochemistry and pharmacology.

What is an Assay?

An assay is the analysis of a component of a sample. The type of measurement can be either qualitative or quantitative. That means; an assay measures the presence, amount or the functional activity of a component in a sample.
Assay methods normally incorporate accurate and precise analytical methods. There are various types of assays. Depending on the sample to be analyzed and the other requirements, you can choose the most appropriate assay type. Chemical
assays are done using techniques like chromatography, titrations, etc.
Bioassays are another type of assays carried out to quantify the effect of samples in biological systems. It includes studies of drugs on micro-organisms, virulence studies on human, bioassay of hormones, etc.
On the contrary, assays are a type of widely used analyzing methods of samples in pharmacology, laboratory medicine, biochemistry, biotechnology, immunology, environmental biology, etc. Here, the analyte or the main target component can be a drug, biochemical substance, a cell of an organism or an organic sample.
Moreover, assays can be classified based on different aspects. Based on the type and number of measurements taken, there are two types of assays; they are endpoint assays, which measure signals after a fixed incubation time, and kinetic assays, which take measurements multiple times over a fixed time intervals.

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Also, based on the number of analytes detected, there are two types of assays. They are the single target assays, which measure a single component at a time, and multiplex assays, which measure several components simultaneously during the same assay.

It is calculated against standard. It may be of following types:

On as is basis = (Area of sample / Area of standard) x (conc. of standard / conc. of sample) x potency or assay of standard
On anhydrous basis = (Assay on as is basis / 100 – moisture) x 100
On dried basis = (Assay on as is basis / 100 – LOD) x 100

What is Potency?

Potency is a measure of drug activity expressed in terms of the amount required to produce an effect of given intensity. A highly potent drug evokes a given response at low concentrations, while a drug of lower potency evokes the same response only at higher concentrations.
The potency of a drug depends on two factors;

Affinity – this describes the attachment of a drug with a receptor
Efficacy – this describes how well a response is given after attachment of a drug with a receptor

The response given for a drug is the effect of the drug on our body. This effect depends on both the binding of the drug with the receptor (affinity) and the production of the response after the attachment of the drug with a receptor (efficacy).