Laboratory quality
Sterility
The distinct assay that establishes absence of viable microorganisms, and why sterility is independent of purity and identity.
- Last updated:
- 2026-07-22
Overview
Sterility is the absence of viable microorganisms. Sterility is confirmed by an assay: a sample is cultured in growth media that support a wide range of bacteria and fungi, and the media are inspected over a defined period (typically 14 days). No growth means the sample passed.
For pharmaceutical products, sterility testing is regulated and follows methods such as USP <71>. For research-supply peptides, sterility testing is optional, and most CoAs do not include it.
Why it matters
A peptide that has been synthesised, purified, and characterised can still be contaminated with bacteria introduced during fill, storage, or handling. Contamination is invisible in the vial and does not affect the HPLC purity number.
Key concepts
Sterility vs. sterilisation
"Sterile" describes a state; "sterilised" describes a process. A product can be sterilised (e.g. by 0.22 µm filtration) and still fail sterility testing if contamination occurs after the sterilisation step.
Culture-based limits
Standard sterility testing detects organisms that grow in the chosen media at the chosen temperature. Slow-growing or fastidious organisms may not be detected.
Sample size
Sterility is tested on a subset of vials from a batch. A pass on the tested vials does not prove every vial in the batch is sterile.
Common misconceptions
- "Sterile-filtered" is not the same as "sterile." Filtration reduces microbial load and can produce sterile product if performed correctly, but the resulting product still needs a sterility test to confirm the outcome.
- "Visually clear" is not sterile. Contamination that matters is usually invisible.
- Sterility does not imply low endotoxin. Dead bacteria still shed endotoxin.
Practical interpretation
Where a research vial does not carry a sterility statement, treat it as an unknown, not as sterile. Where it does, note the method and the passage criteria.
Limitations
Sterility testing is destructive; the tested vials are consumed. Extrapolation to the untested vials in the batch is a statistical inference, not a guarantee.