Endotoxin and sterility: two questions, often confused

Sterility asks whether viable organisms are present now. Endotoxin testing asks whether the residue of bacterial growth is present, regardless of whether anything is still alive.

Those come apart because endotoxin is remarkably durable. It is a fragment of the outer membrane of gram-negative bacteria, and it is not a living thing — it is a molecule. Killing the organism does not remove it. It survives autoclaving, and it passes straight through the filters used to sterilize a solution.

So a sample can be perfectly sterile and carry a substantial endotoxin figure at the same time. If bacteria grew in the material at some earlier point and were subsequently removed or killed, the sterility test reports what you hoped and the endotoxin test reports what happened.

The classical method is the LAL assay, which uses a clotting reaction derived from horseshoe crab blood. Endotoxin triggers the cascade, and the degree of reaction is read against a standard curve.

The result is reported in endotoxin units, usually normalized to mass as EU/mg. Because it is defined against a reference standard rather than in absolute molecular terms, an endotoxin figure is only meaningful alongside the method that produced it.

A newer alternative, recombinant Factor C — rFC — reproduces the first step of the same cascade using a manufactured enzyme rather than a blood-derived reagent. It measures the same thing without depending on horseshoe crab harvest, and it is increasingly accepted where the older assay was once the only option. A certificate reporting both is showing its work.

A sterility test incubates material in growth media and looks for anything that multiplies. It is a direct question with a slow answer: the incubation period is measured in days, because slow-growing organisms are the ones a short test would miss.

Its limitation is statistical rather than technical. Sterility testing samples a portion of a batch, so a pass is evidence about the batch rather than a guarantee about every container in it. This is why process control matters more than final testing, and why a supplier's answer about how material is handled is worth as much as the certificate.

Three things are worth checking, and only the first is usually printed in large type.

Endotoxin and sterility testing cost money, take time, and can only lower the impression a certificate gives. A supplier who tests and publishes is accepting a number that might have been better left unmeasured.

That is precisely what makes the presence of these figures informative. Purity is the number everyone volunteers. Endotoxin is the number that has to be asked for.