Endotoxin and sterility get mentioned in the same breath a lot, and it's easy to assume they're the same test. They aren't. They measure different things, by different methods, and a result on one tells you nothing about the other.
Endotoxin: a chemical measurement
Endotoxins are pieces of the outer wall of certain bacteria. They stay in a sample even after the bacteria that made them are no longer alive. Endotoxin testing measures how much of this material is in the sample, most often with a test called an LAL assay (it uses a substance drawn from horseshoe crab blood that reacts to endotoxin in a way a lab can measure). The result is a number, checked against a stated limit, not a simple pass or fail.
Because endotoxin is a leftover chemical rather than a living thing, a sample can test low for endotoxin while still containing live bacteria that haven't produced detectable endotoxin yet, or already shed what they made. That is exactly why the two tests are run and reported separately.
Sterility: a biological measurement
Sterility testing asks a different question: does this sample show any live microbial growth. The standard method places a sample in a growth medium and watches it over a set period. Growth is a fail; no growth is a pass.
Sterility testing uses up the sample: testing it means it can't be sold afterward. So a sterility result always comes from part of the lot, not from the exact vial in your hand. It supports confidence in the lot as a whole. It is not a guarantee for every single vial.
Why one does not mean the other
- A low endotoxin reading does not mean a sample is sterile. Endotoxin is a leftover, not a count of living organisms.
- A sterile result does not mean a sample has no endotoxin. Bacteria that are no longer alive can still leave endotoxin behind, even when nothing grows in the sterility test.
A certificate that reports both is answering two separate questions with two separate tests. A certificate that reports only one is silent on the other; it isn't quietly passing it too. See reading a certificate of analysis for how to check which tests actually ran on a given lot.
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