Potency Assay

A potency assay measures what a molecule does, not how much of it is present. The readout is biological: a cell line engineered to express the target receptor reports second-messenger accumulation or reporter-gene output, a binding assay measures competition against a labelled ligand, or an animal bioassay measures a physiological response. The result is inherently relative, expressed against a reference standard run in the same experiment rather than as an absolute quantity.

This is how biological products were controlled long before modern analytics existed. Insulin was standardised in units by a rabbit blood-glucose bioassay decades before physicochemical methods could do the job, and somatropin potency is still assessed by cell-based methods. Release specifications for potency are conventionally written as a window around the label claim rather than a single value, because the assays themselves carry substantial run-to-run variability.

Potency catches failures that chromatography and mass spectrometry cannot. Scrambled disulfides, partial unfolding, soluble aggregates and stereochemical errors can leave retention time and molecular mass essentially untouched while destroying activity. That makes potency the release test that connects a specification to the reason anyone cares about the molecule, and it is why it appears on regulated product specifications and almost never on a research-chemical certificate.

Two misreadings are common. The first is treating a purity figure as a potency figure; they are unrelated numbers, and an unremarkable chromatogram can sit alongside biologically dead material. The second is over-interpreting small differences: cell-based potency assays vary by tens of percent between runs, so a result quoted without a reference standard, replicate count or acceptance range is not interpretable, however precise the number looks.