Sterility and endotoxin are different questions
Two tests, two questions, and one does not imply the other in either direction. A sterile solution can be pyrogenic, and a contaminated one can read clean for endotoxin.
The two questions
Sterility asks whether viable organisms are present. It is answered by putting the sample into growth media, incubating for fourteen days, and looking for growth.
Endotoxin asks whether lipopolysaccharide is present. It is answered by an enzymatic or cell-based assay run in hours, and it is indifferent to whether the bacteria that produced the material are alive, dead or were removed weeks ago.
Mixing the two up is the most common mistake in reading this kind of paperwork, and it runs both directions.
The four combinations
Sterile and low endotoxin
What a well-made preparation looks like. Both tests were run, both passed, and both results appear on the document with their methods.
Sterile, high endotoxin
Entirely possible and the case that catches people. The solution was filtered or autoclaved, so nothing grows. The lipopolysaccharide from the bacteria that were there beforehand passed the filter or survived the heat, and it is still in the vial.
Non-sterile, low endotoxin
Also possible. Gram-positive organisms and fungi do not produce lipopolysaccharide, so a contaminated preparation can read clean on an endotoxin assay while growing something the sterility test would find.
Neither tested
The most common state for research material, and not by itself a defect. It becomes a defect when a certificate uses the word sterile without a test behind it.
How the two tests differ in practice
- Sterility, method
- Membrane filtration or direct inoculation into growth media
- Sterility, duration
- 14 days incubation, so it cannot be run on a rush
- Sterility, detects
- Viable bacteria and fungi, gram-positive and gram-negative alike
- Endotoxin, method
- LAL, rFC or MAT
- Endotoxin, duration
- Hours
- Endotoxin, detects
- Lipopolysaccharide, from gram-negative organisms only, alive or dead
What lyophilized material can and cannot be said to be
Lyophilization removes water, which stops most organisms growing. It does not kill everything present, and spores in particular survive it comfortably. A dry powder is therefore not sterile by virtue of being dry, though it is a poor environment for anything to multiply in.
It also does nothing whatsoever to endotoxin. Whatever lipopolysaccharide was in the solution before freeze drying is in the cake afterwards, concentrated by exactly the factor the water removal implies.
So neither claim follows from freeze-drying. Each needs its own test, and where a test has not been run, the honest statement is that it has not been run.
Questions
Common questions
Is lyophilized peptide sterile?
Not by default. Freeze drying prevents growth and does not achieve sterility, and spores survive it. Sterility is a claim that requires a fourteen day sterility test behind it.
If a supplier tests endotoxin, have they tested sterility?
No, and the reverse is equally untrue. They are separate tests on separate instruments with separate costs, and it is common to see one without the other. Look for both named explicitly.
Why does the sterility test take fourteen days?
Slow-growing and stressed organisms need time to become detectable. A shorter incubation produces a result that has not given everything present a chance to grow, which is why the period is fixed rather than left to judgment.
Can a solution be made non-pyrogenic after the fact?
Endotoxin can be reduced by anion exchange, ultrafiltration or affinity resins, and how well any of them works depends on the molecule being purified alongside it. It is a real process and not a trivial one, and removal is harder than never introducing it.
Cite as: SBz Industries research library, "Sterility and endotoxin are different questions", https://www.sbzindustries.com/research/sterility-vs-endotoxin, updated 2026-09-11.
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