Endotoxin Testing Explained
What endotoxins are, why they matter in peptide quality, and how to interpret endotoxin test results.
Last updated: 14 April 2026
What are endotoxins?
Endotoxins are lipopolysaccharides (LPS) found in the outer membrane of gram-negative bacteria. Even in tiny amounts, endotoxins can trigger immune responses in the body. Manufacturing and handling environments can introduce gram-negative bacteria.
Peptides, especially those from bacterially derived sources or manufactured in non-sterile settings, may contain endotoxin contamination.
Why endotoxin testing matters
Endotoxins are pyrogenic — they cause fever and immune activation. For injectable peptides, endotoxin levels must be extremely low. USP (US Pharmacopeial Convention) and PhEur (European Pharmacopoeia) set limits for injectable goods.
Even for non-injectable use, high endotoxin loads are a quality concern.
Testing methods
The LAL (Limulus amebocyte lysate) test is the standard. It detects endotoxins at levels as low as 0.001–0.1 EU/mL depending on the assay used. Results are reported in Endotoxin Units per mL (EU/mL) or per sample.
Other methods include recombinant versions of the LAL test. A good supplier will specify which test was used.
Interpreting results
For injectable peptides, acceptable endotoxin levels are typically <0.25 EU/vial or <5 EU/gram. Non-injectable peptides may tolerate higher levels, but <5 EU/gram is a reasonable standard.
Check the supplier's report for the test method, the limit of detection, and the result. 'Below detection limit' is good; a high number is concerning.