USP General Chapter 85 (revision 28)

USP General Chapter 85, titled Bacterial Endotoxins Test, is the chapter of the United States Pharmacopeia that prescribes how endotoxin is measured, how the limit for a product is calculated, and how method suitability is demonstrated.^[1]

Chapters numbered below 1000 are mandatory where a monograph or another applicable requirement references them; chapters numbered 1000 and above are informational. This distinction matters when reading a certificate that cites a chapter, because citing an informational chapter is a different kind of claim from citing a mandatory one.^[2]

The chapter is harmonised in substance with the corresponding chapters of the European and Japanese pharmacopoeias, so a determination described as conforming to one is broadly comparable with one conforming to another.^[1]

The chapter defines the three permitted techniques — gel-clot, turbidimetric and chromogenic — and treats the gel-clot method as the referee method where results conflict. It specifies preparation of the standard endotoxin, the confirmation of lysate sensitivity, and the inhibition and enhancement testing that establishes whether a particular sample matrix interferes with the reaction.^[1]

It also specifies how the endotoxin limit for a product is calculated. The limit derives from the threshold pyrogenic dose and the maximum human dose per kilogram per hour, so a limit expressed per milligram of substance depends on the dose that substance is given at. Two products with the same per-milligram figure may sit on opposite sides of their respective limits.

The maximum valid dilution follows from the limit and the lysate sensitivity, and constrains how far a sample may be diluted to overcome interference. Dilution is the usual remedy for interference, and the chapter bounds it so that dilution cannot be used to dilute the endotoxin below detection as well.^[1]

A certificate stating "endotoxin: <0.5 EU/mg, USP <85>" is asserting that the determination followed the chapter. The fields that make that assertion checkable are the method variant used, the lysate sensitivity, and whether inhibition and enhancement testing was performed on this matrix.^[1]

A numeric result reported from a gel-clot determination is a category error, since the gel-clot method reports whether a clot formed at a dilution rather than a concentration. Certificates showing a number attributed to a gel-clot method are internally inconsistent — see Test report forgery for the general treatment of internal consistency.^[3] Where a non-endotoxin pyrogen is at issue, a different chapter applies entirely.^[4]

Compendial chapters apply to articles for which a monograph or a regulatory requirement invokes them. Research-chemical peptides are not such articles, so endotoxin determination on them is voluntary and the compendial limits do not formally apply.^[2]

Citing the chapter on such a certificate is nonetheless meaningful in a narrower sense: it says the method followed a published procedure rather than an internal one, which makes it comparable with other determinations. That is a statement about method provenance, not about compliance.

The distinction between "tested by the method of USP <85>" and "meets the USP <85> limit for a parenteral product" is substantial, and the two are not always kept apart in supplier documentation. A reader should establish which is being claimed.^[3]

• Bacterial endotoxin test
• USP General Chapter 71
• Sterility testing
• Certificate of analysis
• Limit of detection