Endotoxin and Sterility Testing Explained

Two separate questions

A sterility test asks whether viable microorganisms are present. An endotoxin test asks whether bacterial lipopolysaccharide is present. A sample can be perfectly sterile and still carry high endotoxin, because endotoxin is a heat-stable cell wall fragment that survives the death of the organism that produced it.

LAL and recombinant Factor C

The classical endotoxin assay uses limulus amebocyte lysate, derived from horseshoe crab blood, which clots in the presence of endotoxin. Gel-clot, turbidimetric and chromogenic formats trade simplicity for quantitative precision.

Recombinant Factor C assays reproduce the first enzyme of that cascade in a manufactured reagent. They avoid the animal-derived supply chain and are now recognised in major pharmacopoeias.

Results are reported in endotoxin units per millilitre or per milligram, and the acceptable limit depends entirely on the intended application of the material.

USP <71> sterility testing

The compendial sterility test incubates the sample in growth media for fourteen days across two media types to catch aerobes, anaerobes and fungi. It is a destructive test on a sample of the batch, so it demonstrates the absence of detected growth in what was tested rather than proving the whole lot is sterile.

For research-use material, the relevant point is that these tests characterise a lot; they do not make an unapproved compound suitable for any use in humans or animals.