What Is Endotoxin Testing on a Peptide COA?
Endotoxin testing detects bacterial contamination that HPLC purity cannot. Here is what it measures, why it matters, and how to read the result.
Endotoxin testing detects bacterial endotoxin contamination in research peptides. Unlike HPLC purity, endotoxin testing measures something HPLC cannot see: leftover components from bacteria that grew on or near the synthesis pathway. It is one of the most important quality tests on a peptide Certificate of Analysis, and it is often missing.
What endotoxin is and where it comes from
Endotoxin is a fragment of the outer membrane of Gram-negative bacteria. Even after bacteria are killed by sterilization, the endotoxin remains and can cause inflammatory reactions in biological systems. In peptide synthesis, endotoxin can come from contaminated reagents, water, or equipment surfaces.
How endotoxin is tested (USP 85)
The standard pharmacopeial method is USP Chapter 85, the Limulus Amebocyte Lysate (LAL) assay. A sample of dissolved peptide is mixed with a reagent derived from horseshoe crab blood. If endotoxin is present, the reagent gels or develops color, depending on the assay format. Results are reported in Endotoxin Units (EU) per milligram of peptide.
How to read an endotoxin result
Results are reported in endotoxin units (EU), usually per mg of peptide or per mL of solution. Lower is better. Compare the reported value with the specification printed on the same certificate, and check the units match before comparing across vendors.
Which vendors run endotoxin testing
Endotoxin testing is the test that separates premium vendors from average ones. It costs the vendor more per batch, but it is the only way to confirm the peptide is not contaminated with bacterial fragments. Among vendors we re-audited in October 2026, Direct Peptides, Felix Chemical Supply, Onyx BioLabs, Prime Peptides and Protide Health publish endotoxin results on certificates we confirmed with the issuing laboratory. Each vendor page lists which of its certificates include an endotoxin result.
Frequently Asked Questions
Why does HPLC purity not catch endotoxin contamination?
Endotoxin molecules do not absorb in the standard UV range used by HPLC purity detection. The compound is invisible to that method. Endotoxin requires its own dedicated LAL assay (USP 85).
Does sterility testing replace endotoxin testing?
No. Sterility testing detects live microorganisms. Endotoxin testing detects bacterial fragments left behind after sterilization. Both tests are needed for a complete contamination panel.
Is endotoxin testing required by law for research peptides?
No. Research peptides are sold for laboratory use only and are not subject to the same FDA requirements as injectable pharmaceuticals. The presence or absence of endotoxin testing is the vendor's decision and is a quality differentiator.