Guides / Endotoxin limits explained
How the 5 EU/kg/hour endotoxin limit is calculated
What endotoxin limit applies to an injectable product?
The endotoxin limit is K divided by M, where K is 5 endotoxin units per kilogram per hour for parenteral routes and 0.2 EU/kg for intrathecal, and M is the maximum dose per kilogram per hour. For a 70 kilogram subject the total permitted parenteral load is 350 EU per hour of administration. A product with no defined dose has no calculable limit, and the result is reported as a concentration rather than as a pass.
Start from the threshold pyrogenic dose. K is 5 EU per kilogram per hour for intravenous, intramuscular and subcutaneous routes, and 0.2 EU/kg for intrathecal, where the twenty-five-fold tightening reflects the absence of the systemic clearance available elsewhere in the body.
M is the maximum human dose administered per kilogram in one hour. Divide K by M and the result is the permitted endotoxin concentration per unit of product, expressed in whatever unit the dose was expressed in.
For a 70 kilogram subject receiving a single vial, the arithmetic is straightforward: 5 EU/kg times 70 kg gives 350 EU permitted for that hour. If the vial delivers the entire dose, the vial limit is 350 EU. If the product is dosed at 2 mg per kilogram and a vial holds 10 mg, the limit per milligram is 5 divided by 2, that is 2.5 EU per milligram, so 25 EU per vial.
| Route | K value | Note |
|---|---|---|
| Intravenous, intramuscular, subcutaneous | 5 EU/kg/h | Standard parenteral |
| Intrathecal | 0.2 EU/kg | Twenty-five-fold tighter |
| Radiopharmaceutical, intravenous | 175 EU/V | Per volume rather than per weight |
| Ophthalmic, intraocular | Product specific | Assessed case by case |
Peptides, surfactants, chelators and buffers interfere with the amoebocyte cascade, and they do so in both directions. Inhibition suppresses the signal and returns a falsely clean result. Enhancement inflates it. Neither is visible from the number itself.
The control is a spiked recovery: standard endotoxin is added to the sample at a known concentration and the assay must return between 50 and 200 percent of it. Outside that window the matrix is interfering and the determination is not valid at that dilution.
The remedy is dilution, which reduces interference but also raises the detection limit. Above the maximum valid dilution the assay can no longer see down to the limit being tested, and the determination fails on sensitivity rather than on the sample. Both the recovery figure and the dilution factor therefore belong on the certificate; a result without them cannot be assessed by the reader.
Endotoxin is a heat-stable lipopolysaccharide. Autoclaving kills the organism and leaves the molecule. A 0.22 micron sterilising filter removes cells and passes lipopolysaccharide freely.
A preparation can therefore pass a sterility test and remain pyrogenic, which is why the two determinations are separate and why depyrogenation, typically dry heat at 250 degrees, is a distinct process step from sterilisation.
Gel-clot is a limit test giving presence or absence at a labelled sensitivity. It is inexpensive and robust and returns no number.
Kinetic chromogenic and kinetic turbidimetric methods give quantitative results against a standard curve and are the practical choice wherever the figure matters rather than merely the pass.
Recombinant Factor C, official as USP <86> since May 2025, replaces horseshoe crab lysate with a recombinant protein. It removes an animal-derived supply chain and, because it relies on one defined enzyme rather than a three-factor cascade, it is less susceptible to certain interferences.
What is the endotoxin limit for an injectable?
5 endotoxin units per kilogram of body weight per hour for standard parenteral routes. For a 70 kilogram subject that is 350 EU per hour of administration. Intrathecal products are limited to 0.2 EU/kg.
Can a sterile product contain endotoxin?
Yes. Endotoxin is a heat-stable molecule rather than a living organism. It survives autoclaving and passes through sterilising filters, so sterility and pyrogenicity are independent.
What is spike recovery and why does it appear on the certificate?
A known amount of standard endotoxin is added to the sample; the assay must recover between 50 and 200 percent of it. Without that figure there is no evidence the assay functioned on that matrix, and a clean result may simply be suppressed.
What is maximum valid dilution?
The largest dilution at which the assay can still detect endotoxin at the product's limit. Diluting further reduces interference but pushes the detection limit above the limit being tested, invalidating the determination.
References
- USP <85>, Bacterial Endotoxins Test
- USP <86>, Bacterial Endotoxins Test Using Recombinant Factor C
- European Pharmacopoeia 2.6.14
Sources
Updated 2026-08-24