Testing methods

Sterility and Endotoxin Testing: What the Terms Actually Mean

Published October 2026 · 6 min read

Product pages sometimes use "sterile," "low endotoxin" or "microbiologically tested" as if they were interchangeable. They describe different things, they are measured by different methods, and a claim without a method and a number behind it isn't one you can evaluate.

Sterility

Sterile means free of viable microorganisms. It is tested by culture-based methods — membrane filtration or direct inoculation into growth media, incubated for about two weeks — as described in pharmacopoeial chapters such as USP <71>. Sterility is also a statement about how a material was made and handled (for example, aseptic filling), not just a lab result. A sample passing a test doesn't make an unsterile manufacturing process sterile.

Bioburden

Bioburden is a count of viable microbes (colony-forming units) in a material that is not claimed to be sterile. It is a measure of cleanliness, not a guarantee of sterility.

Endotoxin

Endotoxins are lipopolysaccharide fragments from the cell walls of Gram-negative bacteria. They are heat-stable and are not removed by simply killing the bacteria, so a sample can be sterile and still contain endotoxin. In cell culture they matter because immune cells respond to extremely small amounts, which can mimic or mask the effect you are trying to study.

The standard measurement is the LAL assay, which uses a lysate from horseshoe crab blood (gel-clot, turbidimetric and chromogenic formats exist), along with recombinant Factor C assays as an alternative. Results are reported in endotoxin units (EU) per milligram or per milliliter, against a stated limit.

Why "sterile" doesn't imply "endotoxin-free"

Filtration through a sterilizing-grade membrane removes bacteria, but endotoxin pieces are far smaller and can pass through or already be in the material. They have to be controlled at the source and measured separately.

What to expect on research-grade paperwork

Research materials are generally not manufactured to pharmaceutical standards, and most COAs don't include sterility or endotoxin results at all. That isn't deceptive in itself; it just means the data doesn't exist. Materials sold for research use are not intended for human or veterinary use, and a sterility or endotoxin statement doesn't change that.

If a listing does say "sterile" or "endotoxin-tested," these are the details that make the claim usable:

• The method (for example USP <71>, or LAL gel-clot / chromogenic).
• The result with units and the limit it was compared against.
• The lot it applies to and the lab that ran it.

Related reading

• Peptide COA Red Flags: Ten Things to Check
• Peptide Testing Labs: How to Check the Lab on a COA
• Research peptide glossary

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