Endotoxin testing: the LAL assay.
Bacterial endotoxins are heat-stable fragments of Gram-negative bacterial cell walls. Unlike live bacteria, they survive sterilization and can cause fever and inflammatory reactions at tiny doses. The LAL (limulus amebocyte lysate) assay detects them, and results are reported in endotoxin units per milliliter (EU/mL) on the COA.
What endotoxins are
Endotoxins are lipopolysaccharide (LPS) components of the outer cell wall of Gram-negative bacteria. They are shed when bacteria die — and critically, they are not destroyed by sterilization, filtration, or typical peptide synthesis conditions. A product can be perfectly sterile and still carry concerning endotoxin loads.
Why it matters for research peptides
Peptides are manufactured and handled in environments where bacterial contamination is possible — water, buffers, lyophilization steps. Because endotoxins are stable and biologically active at extremely low concentrations, they are the classic hidden hazard of injectable research materials.
The LAL assay
The industry-standard test is the Limulus Amebocyte Lysate (LAL) assay, built on a blood protein from the horseshoe crab that clots in the presence of endotoxin. Variants include gel-clot, kinetic turbidimetric, and chromogenic methods — all quantifying endotoxin in endotoxin units per milliliter (EU/mL).
Reading the result on a COA
- Look for the numeric result (e.g., "<0.5 EU/mL") and the method.
- The relevant limit depends on the intended use; pharmacopeial references define endotoxin limits for injectable products (USP <85> / EP 2.6.14 frameworks).
- A COA with no endotoxin result for an injectable-intended research peptide is missing its most safety-relevant number — see how to read a peptide COA.
Endotoxin vs sterility — the two are different
Sterility testing asks whether live microorganisms are present. Endotoxin testing asks whether the dead-bacterial debris remains — which sterility testing cannot address. A complete safety panel includes both, plus heavy metals where synthesis is a concern.
Questions. answered.
What is endotoxin testing?
A laboratory assay (usually LAL) that detects and quantifies bacterial endotoxin — the heat-stable cell-wall debris of Gram-negative bacteria — reported in EU/mL.
Does sterilization remove endotoxin?
No. Endotoxins survive autoclaving and standard sterilization. Sterility testing and endotoxin testing measure different hazards, which is why COAs list both.