Cell viability, identity, sterility and potency explained
These four words appear on every certificate of analysis and almost never get explained to patients. Each answers a different question: are the cells alive, are they the right cells, is the product clean, and do the cells still function?
Viability — are the cells alive?
Viability is the percentage of intact, living cells in the dose. It is measured by dye exclusion or flow cytometry, and it matters twice: at release, and again after thawing, because freezing and thawing kill some cells. A dose quoted as "100 million cells" means little without the viability figure and the time between thaw and administration.
What do viability, identity, sterility and potency actually measure?
Viability is the percentage of living cells at release; identity confirms the cells carry the expected mesenchymal surface markers; sterility confirms no bacterial or fungal growth; potency attempts to measure biological activity, usually through immunomodulation assays. The first three are standardised and routine. Potency assays remain the least standardised part of cell therapy quality control worldwide.
Key facts
- Viability
- % living cells at release
- Identity
- Surface-marker immunophenotype
- Sterility
- No microbial growth in culture
- Potency
- Functional assay, not yet standardised field-wide
What we know
- Viability, identity and sterility have accepted methods and thresholds.
What remains uncertain
- No consensus potency assay exists for MSCs, so cross-laboratory potency comparisons are unreliable.
Medically reviewed questions
This summary is general medical information, not medical advice, and not a promise of benefit. Figures are quoted ranges, confirmed in writing after a medical review.
Medically reviewed by: Prof. Dr. Erdal Karaöz · Last updated: · Editorial and evidence policy
Identity — are they the right cells?
Identity confirms the product is a mesenchymal stromal cell population and not something else that grew in culture. The standard reference is the ISCT minimal criteria: plastic adherence, expression of CD73, CD90 and CD105, absence of haematopoietic markers such as CD34, CD45 and HLA-DR, and demonstrated trilineage differentiation capacity. Ask which markers were measured and at what percentage.
Sterility, endotoxin and mycoplasma — is it clean?
Contamination is the failure mode with the clearest link to patient harm in the published record of unregulated clinics, which is why these three are non-negotiable.
- Sterility — validated culture-based testing for bacterial and fungal growth.
- Endotoxin — measured in endotoxin units per kilogram; a product can be sterile and still cause a fever reaction if endotoxin is high.
- Mycoplasma — small bacteria that pass standard filters and do not show on routine sterility tests; tested by PCR or culture.
Potency — do they still work?
Potency is a functional assay: for MSCs, commonly the ability to suppress activated T-cell proliferation, or measured secretion of a relevant factor such as IDO or PGE2 after stimulation. Cells that have been expanded too far can pass identity and viability while losing function, so potency is what distinguishes a controlled product from a cell count.
What a specification looks like
A failing batch means your treatment is postponed. A provider who has never postponed a treatment for a failed release is worth a second look.
| Test | Typical specification format | If it fails |
|---|---|---|
| Viability | ≥ defined % at release and post-thaw | Batch rejected or dose recalculated |
| Identity | ≥ defined % positive for CD73/CD90/CD105; ≤ defined % for CD34/CD45 | Batch rejected |
| Sterility | No growth | Batch destroyed |
| Endotoxin | Below EU/kg limit | Batch destroyed |
| Mycoplasma | Not detected | Batch destroyed |
| Potency | Assay result within a defined range | Batch rejected |
Frequently asked questions
Is a higher cell count always better?
No. Dose is chosen per protocol and per kilogram of body weight where relevant; beyond a point, more cells add cost and infusion load rather than benefit, and viability and potency matter more than the headline number.
What is passage number and why does it matter?
Passage number counts how many times cells have been subcultured. Higher passages yield more cells but risk senescence and loss of potency, which is why passage limits are written into the process.
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