What Is a "Potency Assay"? How Scientists Confirm a Cell Therapy Actually Does Its Job

What This Article Covers

Before any cell-based therapy can be given to a patient — whether in an FDA-approved product or a rigorously run clinical trial — scientists have to answer four separate questions about it: Is it the right cells? Is it clean? Is it safe? And does it actually work? That last question is answered by something called a potency assay, one of the least publicly understood but most consequential steps in cell therapy development. This article explains what a potency assay is, why it's uniquely difficult to build for living cells, and why its presence (or absence) is one of the clearest signals of how rigorously a cell-based product has actually been developed.

The Four Questions Every Biological Product Must Answer

Under U.S. federal regulations governing biological products, every batch, or "lot," of a cell or gene therapy product must be evaluated against four criteria before it can be used: identity, purity, safety, and potency. Identity confirms the product is actually made of the cells it claims to be. Purity confirms it isn't contaminated with the wrong cell types, residual chemicals, or other unwanted material. Safety testing screens for things like infectious agents or toxic byproducts. Potency is different from all three — it asks whether the product has the biological activity it's supposed to have, at a level strong enough to produce the intended effect.

The FDA defines potency, in regulatory language, as "the specific ability or capacity of the product… to effect a given result." In plainer terms: a potency assay is a laboratory test built to answer, "Will this particular batch actually do the thing it's designed to do?"

Why "Is It a Stem Cell?" Isn't the Same Question as "Will It Work?"

It's easy to conflate potency with the identity testing many readers may already be familiar with — the surface-marker panels and differentiation tests used to confirm a cell population qualifies as a mesenchymal stem cell, for example. But those tests only establish what a cell is. They don't establish what a specific manufactured batch of that cell will do in a patient.

Two lots of cells can both pass identity testing — same markers, same general classification — and still differ meaningfully in how much of a therapeutic protein they secrete, how effectively they home to an injury site, or how strongly they modulate immune activity. Potency testing is built to catch exactly that kind of batch-to-batch variability, which is one of the defining challenges of working with living material rather than a fixed chemical compound.

Why Potency Is Unusually Hard to Measure in Cell Therapies

For a conventional small-molecule drug, potency is often straightforward to define: a specific molecule either binds its target or it doesn't, at a specific measurable strength. Cell therapies are a different kind of problem. A cell population is heterogeneous, its behavior can shift with each manufacturing run, and in many cases scientists don't have complete certainty about the precise mechanism by which the cells produce their therapeutic effect in the body.

That last point is not a minor technicality — it is arguably the central difficulty of the field. If regulators and manufacturers aren't entirely sure how a cell therapy works, it becomes genuinely hard to design a lab test that reliably predicts how well it will work. This is an active area of methods development: the FDA issued its original guidance on potency testing for cellular and gene therapy products in January 2011, and followed it with additional draft guidance in late 2023 specifically aimed at helping developers build more robust, mechanistically grounded potency assays — a sign that the science of measuring potency is still maturing even as the underlying cell therapies themselves advance.

How Potency Testing Changes as a Product Moves Through Development

Regulators don't expect a fully polished potency assay to exist on day one of a clinical trial — but they do expect it to keep improving. In early-phase trials, developers may rely on more limited or qualitative measures of biological activity, with wider acceptance ranges, since the assay itself is often still being refined alongside the therapy. By later-phase trials, a validated, quantitative assay with clearly defined acceptance criteria becomes essential. By the time a product is ready for a full biologics license application — the step required for formal approval — it must include a validated potency assay with defined specifications that every commercial lot has to meet before it can be released for use.

In practice, this means potency testing functions as a gatekeeper. A batch of cells that fails to meet its potency specification isn't supposed to reach a patient, regardless of how it looks on identity or purity testing alone.

Why This Matters When Evaluating Any Cell-Based Treatment

For a newcomer trying to make sense of the stem cell field, potency testing offers a useful, concrete question to ask: does this product have a validated way of demonstrating it does what it claims to do, on a lot-by-lot basis? FDA-approved cell and gene therapy products are required to have this in place. Products offered outside that regulatory pathway — including many procedures marketed directly to consumers — are not held to the same standard, and in many cases no public potency data exists at all.

This doesn't mean every unapproved product is ineffective; it means there is often no independently verified basis for claiming it's effective in the first place. Potency testing is, in essence, the scientific replacement for taking a manufacturer's word for it.

A Genuinely Unsolved Problem, Not Just a Paperwork Requirement

It's worth resisting the temptation to treat potency assays as a bureaucratic checkbox. Building a reliable potency assay forces a research team to nail down, in measurable terms, a mechanism of action that in many cell therapies is still only partially understood. That difficulty is a real and legitimate scientific challenge — one that reputable developers actively invest in solving, and one that continues to draw fresh regulatory guidance because the field hasn't fully solved it yet.

Bottom Line

A potency assay is the test that answers whether a specific batch of a cell therapy will actually produce its intended biological effect — a distinct and often harder question than confirming what the cells are or whether they're free of contamination. Because cell products are biologically variable and their mechanisms of action aren't always fully understood, building reliable potency assays remains one of the genuinely unresolved challenges in the field, and U.S. regulators have continued refining their guidance on the subject as recently as 2023. For anyone evaluating a stem cell-based product or procedure, asking whether a validated potency measure exists is one of the most concrete ways to gauge how rigorously it has actually been developed.