Endotoxin Testing and Sterility Testing: Different Questions

Two different attributes

Sterility testing asks whether viable microorganisms can be recovered from a sample. Endotoxin testing asks how much bacterial lipopolysaccharide is present. A sample can be sterile and still carry a high endotoxin burden, because endotoxin is a heat-stable cell-wall fragment that persists after the bacteria that produced it are dead.

How sterility testing works

Compendial sterility testing incubates the sample, or a filtered membrane of it, in growth media under defined conditions for a defined period, typically 14 days, and observes for growth. Methods include direct inoculation and membrane filtration.

The statistics of sterility testing

Sterility testing is a sampling test. Only a portion of a batch is tested, so a pass result supports a probabilistic conclusion about the batch rather than proving every unit is sterile. Low-level or unevenly distributed contamination can be missed.

How endotoxin testing works

The bacterial endotoxins test commonly uses a limulus amebocyte lysate reagent or a recombinant factor C assay. Formats include gel-clot, turbidimetric, and chromogenic methods. Results are reported in endotoxin units per millilitre or per milligram.

Interferences

Sample matrices can inhibit or enhance the endotoxin reaction, which is why validated methods include a positive product control to demonstrate the assay works in that specific matrix. An uncontrolled result can be falsely low.

Depyrogenation is separate from sterilisation

Autoclaving kills organisms but does not reliably destroy endotoxin. Depyrogenation typically requires dry-heat at higher temperatures or removal by filtration or chromatography. Treating the two processes as interchangeable is a common error.

What to look for on documentation

Check which test was performed, which method format, the specification limit applied, and whether interference or suitability testing was included. A document stating only sterile with no method or limit conveys little.