Key takeaways
- Endotoxins are lipopolysaccharides from Gram-negative bacteria.
- They can interfere with cell-based assays even at low levels.
- The LAL test, described in USP <85>, is the standard method.
- Results are reported in endotoxin units per milligram (EU/mg).
Endotoxins are lipopolysaccharide (LPS) molecules from the outer membrane of Gram-negative bacteria. They are heat-stable and are not removed by standard sterile filtration. In the lab they matter because they can trigger strong responses in cell-based assays and confound results.
How the LAL test works
The limulus amebocyte lysate (LAL) test uses an extract of blood cells from the horseshoe crab. Endotoxin triggers an enzyme cascade in the lysate, and that reaction is measured.
Three LAL methods
| Method | What is measured | Output |
|---|---|---|
| Gel-clot | Whether a firm gel forms | Pass/fail against a limit |
| Kinetic turbidimetric | Increase in turbidity over time | Quantitative result |
| Kinetic chromogenic | Color released from a synthetic substrate | Quantitative result |
Recombinant factor C (rFC) assays are a non-animal alternative that some laboratories now use.
Reading an endotoxin result
- Results are given in EU per milligram of material (EU/mg).
- Check the method and the laboratory named on the report.
- Inhibition or enhancement controls show whether the sample interfered with the test.
- Compare the result with the tolerance of your own assay.
Frequently asked questions
Does sterile mean endotoxin-free?
No. Sterility refers to living microorganisms. Endotoxin can be present in a sterile material.
Why might endotoxin data be missing for a lot?
Not every lot is tested for every attribute. Ask the supplier for the lot number.
Is a lower EU/mg always better for my work?
Lower is generally preferable for cell-based assays. The level that matters depends on your assay.
Disclaimer. This article is for research and educational purposes only. It describes laboratory and analytical practice and is not medical advice. It contains no dosing, administration or human-use guidance. Veridryx products are sold for laboratory research use only and are not for human or veterinary use.
References
- USP General Chapter <85> Bacterial Endotoxins Test. United States Pharmacopeia.
- USP General Chapter <1085> Guidelines on the Endotoxins Test. United States Pharmacopeia.
- European Pharmacopoeia 2.6.14 Bacterial Endotoxins.