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Short answer
Endotoxin is lipopolysaccharide (LPS), a molecule that forms the outer leaflet of the outer membrane of most Gram-negative bacteria. Its lipid A portion drives most of its biological activity. Labs, device makers and drug manufacturers test for it because FDA treats endotoxin as a contaminant to be controlled in products, processing water and components, with limits calculated under USP or AAMI methods.
Definition and scope
In their review, Wang and Quinn in Subcellular Biochemistry (2010), endotoxins are described as the lipopolysaccharides of Gram-negative bacteria. LPS sits in the outer leaflet of the outer membrane, and the lipid A region accounts for most of the bioactivity. Immune cells detect it through Toll-like receptor 4, and LPS structure differs from one bacterial species to another.
For anyone handling lab or medical supplies, one practical point follows from the source material: FDA notes that purified bacterial endotoxins might react differently from native sources of endotoxin, so protocols should consider the source of the endotoxin used in a study (FDA Q&A, question 3).
Endotoxin versus pyrogen
A pyrogen is any substance that can cause a fever response; endotoxin is one kind. FDA’s guidance on pyrogen and endotoxins testing separates the two on purpose. For devices and drug materials, a manufacturer is expected to assess the risk of non-endotoxin pyrogens, and where those are plausible the rabbit pyrogen test (USP <151>) may be more appropriate than an endotoxin assay. For certain biological products the rabbit pyrogen test is required by 21 CFR 610.13(b). FDA also considers the monocyte activation method case by case.
The practical point: a passing endotoxin result addresses endotoxin. It does not by itself answer the question of other pyrogens.
How labs detect and measure it
FDA states that for more than 30 years it has accepted a Limulus Amoebocyte Lysate (LAL) test in place of the rabbit pyrogen test, and its guidance describes the fundamental principles of gel clot, photometric and kinetic methods. The compendial references are USP <85> (Bacterial Endotoxins Test), USP <161> (medical devices, bacterial endotoxin and pyrogen tests) and AAMI ST72. Results are reported in endotoxin units (EU). For the method comparison and a worked calculation, see our guide to LAL endotoxin testing methods; this page does not repeat it.
Where it matters for labs, clinics and supply decisions
Limits come from calculation, not a fixed table
FDA’s current guidance (Edition 2, March 2026, nonbinding) says the endotoxin limit table in the withdrawn 1987 guidance is out of date, and that the appropriate way to set a limit is to use the calculation methods in the USP or AAMI standards.
Device limits quoted by FDA
For devices, FDA’s question 11 gives the following limits, by the body site the device contacts:
| Device contact | Limit stated by FDA |
|---|---|
| Cardiovascular and lymphatic system | 0.5 EU/mL or 20 EU/device |
| Cerebrospinal fluid | 0.06 EU/mL or 2.15 EU/device |
FDA adds that intraocular devices may warrant a lower limit. The lower figure for cerebrospinal fluid contact shows why the intended use of a product, not only its type, sets the endotoxin expectation.
Checklist: where FDA says endotoxin enters the workflow
- Incoming ingredients and components. FDA notes that many firms monitor incoming ingredients and components for endotoxin contamination.
- Processing water. The same answer names processing water specifically. Our guide to laboratory water grades covers the water side.
- Sample storage and handling. FDA’s question 3 says storage and handling of samples can affect endotoxin detection, so a result is only as good as how the sample was kept.
- Documentation. A certificate of analysis is one place a supplier’s test statements appear; what is tested and by which method is stated by the supplier, not assumed.
Sterile-manufacturing expectations are covered in the EU GMP Annex 1 guide, and device-material testing in the FDA biocompatibility guidance guide.
Common misunderstandings
- “Endotoxin and bacteria are the same thing.” Endotoxin is a molecule from the Gram-negative cell envelope, and FDA’s guidance treats it as its own testing question. Sterilization validation, covered in our sterilization guide, is a separate topic.
- “One endotoxin test covers every pyrogen.” FDA’s guidance separates endotoxin from non-endotoxin pyrogens and keeps the rabbit pyrogen test and monocyte activation in view for the latter.
- “There is one universal endotoxin limit.” FDA points to USP or AAMI calculations, and its own device figures differ by contact site.
- “Endotoxin is any bacterial toxin.” In this guide and in the sources cited, endotoxin means the LPS of the Gram-negative outer membrane; the term is tied to that structure.
Frequently asked questions
What is endotoxin in simple terms?
It is lipopolysaccharide from the outer membrane of Gram-negative bacteria. The lipid A part is responsible for most of its biological activity.
What does EU mean in endotoxin results?
EU stands for endotoxin units, the unit in which FDA’s device limits are expressed, for example 0.5 EU/mL or 20 EU/device for devices contacting the cardiovascular and lymphatic system.
Is endotoxin the same as a pyrogen?
No. Endotoxin is one pyrogen. FDA tells manufacturers to assess non-endotoxin pyrogens as well, and the rabbit pyrogen test may suit cases where those are a risk.
Which standards cover endotoxin testing?
FDA’s guidance refers to USP <85>, USP <161> and AAMI ST72, with USP <151> for the rabbit pyrogen test.
Sources
- U.S. FDA, Pyrogen and Endotoxins Testing: Questions and Answers (Edition 2), March 2026, nonbinding; checked 2026-10-29.
- Wang X, Quinn PJ. Endotoxins: lipopolysaccharides of gram-negative bacteria. Subcell Biochem 2010;53:3-25. PubMed 20593260; checked 2026-10-29.
