Endotoxin on a Peptide COA: EU/mg and the LAL Test Explained

The endotoxin line on a peptide certificate of analysis reports how much bacterial endotoxin a laboratory measured in the sample it tested, in endotoxin units per milligram (EU/mg). It is usually measured with a Limulus amebocyte lysate (LAL) assay. The figure is a measurement of one contaminant in one sample from one lot: it says nothing about identity or purity, and without an acceptance limit printed beside it, it doesn't say whether the sample met any specification.

At a glance

What endotoxin is

Endotoxin is lipopolysaccharide, a large molecule that makes up much of the outer membrane of Gram-negative bacteria. Raetz and Whitfield (2002) reviewed its structure: a lipid portion, lipid A, anchors it in the membrane and carries most of its biological activity, and a chain of sugars extends outward from it. Lipid A is recognised by the innate immune receptor TLR4, which is why cultured cells can respond to trace amounts and why endotoxin is measured in materials used in laboratory work.

Endotoxin is a molecule, not an organism. It remains after the bacteria that shed it are dead, and ordinary sterilisation, which kills bacteria, doesn't reliably destroy it. A result for endotoxin therefore says nothing about whether live bacteria are present, and a test for live bacteria says nothing about endotoxin.

How the LAL assay works

Levin and Bang (1964) reported that endotoxin causes the blood of the Atlantic horseshoe crab, Limulus polyphemus, to clot. The test that grew from that finding uses a lysate of the crab's blood cells (amebocytes). Iwanaga (2007) described the mechanism: endotoxin activates factor C, the first enzyme in a cascade of serine proteases that ends by converting coagulogen into a clot. Laboratories read that cascade in three ways:

• Gel-clot. Whether a firm gel forms at a stated lysate sensitivity: a yes-or-no result at each dilution.
• Turbidimetric. How quickly the mixture turns cloudy as the clot forms, read against a standard curve.
• Chromogenic. How quickly the cascade releases a coloured product from a synthetic substrate, also read against a standard curve.

These methods are set out in USP general chapter <85> Bacterial Endotoxins Test and the matching European Pharmacopoeia chapter 2.6.14. Tests that use recombinant reagents instead of lysate, such as recombinant factor C, are covered by European Pharmacopoeia chapter 2.6.32 and USP general chapter <86>.

What EU/mg means

An endotoxin unit measures activity in the assay, calibrated against a reference standard endotoxin. Because endotoxins from different bacteria differ in activity, an EU isn't a fixed weight of LPS. The “per mg” relates the activity measured to the amount of sample tested. A certificate that prints EU/mg alone doesn't say whether that is per milligram of the powder as weighed or per milligram of peptide in it, and the two differ when the powder also contains water, salts or a counterion (see reading a certificate of analysis).

An endotoxin figure has meaning against a specification: a limit set for a material and its intended purpose. The number alone is a measurement, not a pass or a fail.

What can distort an LAL result

• Interference from the sample. A sample can suppress or enhance the reaction. The pharmacopoeial methods include a positive product control, a known amount of endotoxin added to the sample: for the turbidimetric and chromogenic methods, USP <85> accepts recovery of 50% to 200% of the added amount. Dilution is the usual remedy for interference, which is why sensitivity and dilution belong alongside a result.
• Glucans. Some (1→3)-β-D-glucans activate a second pathway in the lysate, through factor G, and can produce a positive reading without endotoxin. Roslansky and Novitsky (1991) found every LAL reagent they tested at least 1,000-fold more sensitive to endotoxin than to the glucans they examined, and glucan-blocking buffers and recombinant factor C reagents avoid that pathway.
• Sensitivity. Every assay has a lowest concentration it can measure. Results at that floor are often written with “<”; a result printed as a plain number doesn't show how close it sits to the floor unless the method's sensitivity is given.

What the certificates on this site report

The certificates in the certificate library were issued by AxisPharm, LLC. Nearly all list “LCMS and LAL Endotoxin Assay” as their methods and print one endotoxin value in an “Endotoxin Testing Value (EU/mg)” table. The VIP 10 mg certificate for lot VIP10VAS0426, for example, prints 0.061. As of October 2026 the values printed across the library run from 0.048 to 0.069 EU/mg, and two certificates report no endotoxin result at all.

The certificates print the value only. They don't state which LAL method was used (gel-clot, turbidimetric or chromogenic), the assay's sensitivity, the dilution, the positive product control recovery, or an acceptance limit. So they don't show how close a value sits to the assay's floor, or whether it met a specification. Each value describes the sample the laboratory tested from one lot; the lot number is what ties a vial to that report (see what a lot number does and doesn't prove), and the lot lookup opens the certificate for a lot.

Regulatory status

An endotoxin result is a quality-control measurement. It confers no regulatory standing and is not a safety assessment or a finding about permitted applications. Materials described here are not approved by the FDA, are supplied for laboratory research use only, and are not for human or animal consumption; see the research use policy.