Methods / Bacterial endotoxins
Bacterial endotoxins
Is the material pyrogenic?
| Preferred technique | Kinetic chromogenic, or recombinant Factor C |
| Controls reported | Spike recovery 50 to 200 percent, dilution factor |
| Parenteral limit | 5 EU per kg per hour |
| Intrathecal limit | 0.2 EU per kg |
| Not offered on | Oils, raw powders, tablets, capsules |
Endotoxin is lipopolysaccharide from the outer membrane of Gram-negative bacteria. It survives autoclaving and passes through a sterilising filter, so a sterile preparation can still be pyrogenic. The compendial limit for parenteral products is 5 endotoxin units per kilogram of body weight per hour, and 0.2 EU/kg for intrathecal use. Determination is kinetic and quantitative rather than a pass-or-fail limit test.
Applied to: Peptides, Injectable preparations. Reference: USP <85>, USP <86>.
Sterility and pyrogenicity are independent properties. Endotoxin is a heat-stable molecule, not an organism. A culture can be killed by autoclaving or removed by a 0.22 micron filter while the lipopolysaccharide it produced remains in solution at full biological potency.
This is the single most consequential misunderstanding in the field. A preparation that passes a sterility test and a preparation that passes an endotoxin test are making two different statements, and neither substitutes for the other.
Gel-clot is a limit test. Lysate and sample are incubated and the tube inverted; a clot that holds indicates endotoxin at or above the labelled sensitivity. It is inexpensive and robust, and it returns presence or absence rather than a number.
Turbidimetric methods track the increasing opacity of the clotting reaction and derive concentration from the time taken to reach a threshold, read against a standard curve. Chromogenic methods substitute a synthetic peptide substrate that releases a coloured product when cleaved by the activated enzyme cascade, and are the most widely used quantitative option.
Recombinant Factor C, official as USP <86> since May 2025, replaces horseshoe crab lysate with a recombinantly expressed protein. It removes an animal-derived supply chain and, because it uses a single defined enzyme rather than a cascade, it is less prone to certain interferences.
| Method | Chapter | Output | Best suited to |
|---|---|---|---|
| Gel-clot | USP <85> A | Presence or absence | Simple limit confirmation |
| Turbidimetric | USP <85> B | Quantitative | Clear, non-interfering matrices |
| Chromogenic | USP <85> C | Quantitative | Routine quantitative work |
| Recombinant Factor C | USP <86> | Quantitative | Animal-free programmes |
Peptides, surfactants, chelators and buffers all perturb the reaction, and they do so in both directions. Inhibition suppresses the signal and produces a falsely clean result; enhancement inflates it. Neither is visible in the raw number.
The control is a spiked recovery: a known quantity of standard endotoxin is added to the sample and the assay must return between 50 and 200 percent of it. Outside that window the matrix is interfering and the sample must be diluted further, up to the maximum valid dilution, above which sensitivity is lost and the determination cannot support the limit being tested.
An endotoxin figure reported without the spike recovery and the dilution factor is not defensible, because there is no way to tell whether the assay was working on that matrix. Both belong on the certificate.
The endotoxin limit for a product is K divided by M, where K is 5 EU per kilogram per hour for parenteral administration and M is the maximum human dose per kilogram per hour. For a 70 kilogram subject the total permitted load is therefore 350 EU per hour of administration, distributed across whatever volume delivers the dose.
Intrathecal administration uses K equal to 0.2 EU/kg, a twenty-five-fold tightening that reflects the absence of the systemic clearance available elsewhere. Where a product has no defined dose, no limit can be calculated and the result is reported as a measured concentration rather than as a pass.
| Living organisms. Endotoxin persists after the bacteria that produced it are dead. |
| Gram-positive contamination, which produces different pyrogens not detected by this assay. |
| Chemical purity, identity or content of the active substance. |
Every method has a blind spot, and the blind spots are why a certificate lists what was not determined alongside what was. A reader who sees only results will read silence as reassurance.
Can a sterile product still contain endotoxin?
Yes, and it commonly does. Endotoxin is a heat-stable lipopolysaccharide that survives autoclaving and passes through sterilising filters. Sterility and pyrogenicity are separate determinations.
What endotoxin limit applies to an injectable?
5 endotoxin units per kilogram of body weight per hour for parenteral administration, and 0.2 EU/kg for intrathecal. For a 70 kilogram subject the parenteral figure works out at 350 EU per hour of administration.
Why does the certificate report a spike recovery?
Because peptide matrices both inhibit and enhance the assay. Without a recovery figure between 50 and 200 percent there is no evidence the assay was functioning on that sample, and a clean result may simply be a suppressed one.
What is recombinant Factor C?
An animal-free alternative to horseshoe crab lysate, official as USP <86> since May 2025. It uses a single recombinant enzyme rather than a cascade, which removes an animal-derived supply chain and reduces certain interferences.
References
- USP <85>, Bacterial Endotoxins Test
- USP <86>, Bacterial Endotoxins Test Using Recombinant Factor C
- European Pharmacopoeia 2.6.14 and 2.6.32
Sources
Related determinations
Updated 2026-09-01