assay

An assay is the quantitative test that answers how much active drug is actually in a sample. Where an identification test asks is this the right substance, the assay asks exactly how much of it is here, reported as a percentage of the label claim or as mass per unit.

Assays are typically run on a pooled or composite sample to estimate the average content of a batch, and the value must fall within the specification range (commonly 90.0–110.0 percent of label claim for a finished product, tighter for a drug substance). Modern assays usually use high-performance liquid chromatography, which separates the active from excipients and degradation products before measuring it, but titration, UV spectrophotometry, and other techniques are also used.

Assay measures the mean amount, not the unit-to-unit spread — that is the job of content uniformity. A batch can have a perfectly acceptable average assay while still failing uniformity because individual units vary too widely, so the two tests are complementary rather than interchangeable.