Endotoxin: what EU/mg means on a certificate

What bacterial endotoxin is, how the LAL and recombinant Factor C methods detect it, why FDA's limit is a ratio rather than a figure in EU/mg, and why a research certificate usually stops short of it.

By Touchstone PeptidesSeptember 22, 20266 minute read

Endotoxin is a measure of bacterial contamination, not of chemical purity, and it is reported in Endotoxin Units. The figure people expect to see, a single FDA limit expressed in EU/mg, does not exist: FDA's limit is a ratio whose denominator depends on how an article would be given, so an EU/mg number on a certificate cannot be judged against a limit until that assumption is supplied. Most research certificates omit the assay, partly for cost and partly because the limit concept does not apply to material sold for bench work.

What endotoxin actually is

Bacterial endotoxins are described by FDA as members of a class of phospholipids called lipopolysaccharides, found in the outer membrane of gram-negative bacteria. They are not living organisms and they are not removed by sterilisation in the ordinary sense. They survive processes that kill the bacteria that produced them, which is why endotoxin is assayed separately from sterility and why a material can be free of viable organisms and still carry endotoxin from an earlier point in its history.

That makes it a process question rather than a chemistry question. Endotoxin in a peptide preparation comes from water, glassware, columns, raw materials and handling, not from the synthesis chemistry itself.

How the assay detects it

The classical method is the Limulus Amebocyte Lysate assay, LAL, prepared from the blood cells of the horseshoe crab. FDA's position on it is long-standing and stated in the current guidance:

For more than 30 years, FDA has accepted the use of a Limulus Amoebocyte Lysate (LAL) test for endotoxins in lieu of the rabbit pyrogens test.FDA, Pyrogen and Endotoxins Testing: Questions and Answers (Edition 2), March 2026

What the reagent contains is an enzyme cascade. Mizumura and colleagues describe the sequence in Innate Immunity: endotoxin first activates the zymogen factor C; activated factor C then activates factor B, which converts the proclotting enzyme into the clotting enzyme; the clotting enzyme cleaves coagulogen into coagulin. In the gel-clot format that final step is read as a clot that does or does not form. Photometric and kinetic formats read the same cascade as turbidity or as colour developing over time, and FDA's guidance describes the compendial documents as setting out the fundamental principles of the gel clot, photometric and kinetic methods.

The cascade has a known blind spot. A second branch of the same lysate, factor G, is activated by (1,3)-β-D-glucan rather than by endotoxin, so glucan carried into a sample can produce a positive reading that has nothing to do with bacteria.

Recombinant reagents were developed against exactly that problem. Reagents built on the cloned factor C gene have been marketed since 2004; they contain recombinant factor C alone rather than the whole cascade and read out through a fluorogenic substrate cleaved by the activated enzyme. Because the reagent has no factor G, the glucan false positive does not arise. FDA permits alternative assays where they are validated and shown to perform at least as well:

Yes, firms may use alternative methods and/or procedures if they provide advantages in terms of accuracy, sensitivity, precision, selectivity, or adaptability to automation or computerized data reduction, and in other special circumstances. Such alternative procedures and methods should be validated as described in the USP General Chapter <1225>, "Validation of Compendial Procedures", and should be shown to achieve equivalent or better results.FDA, Pyrogen and Endotoxins Testing: Questions and Answers (Edition 2), March 2026

Why no single limit exists in EU/mg

The number circulating in vendor copy, usually stated as a 10 EU/mg FDA limit, is not in any FDA document. FDA's endotoxin limit is a calculation, and its terms are defined in the agency's inspection technical guide:

The endotoxin limit is expressed as K/M. K is 5.0 EU/kilogram (kg.), which represents the approximate threshold pyrogen dose for humans and rabbits ... M represents the rabbit pyrogen test dose or the maximum human dose per kilogram that would be administered in a single one hour period, whichever is larger.FDA, Inspection Technical Guide No. 40, Bacterial Endotoxins/Pyrogens, March 20, 1985

K is a constant, 5.0 EU/kg for the general case and 0.2 EU/kg where an article is labelled for intrathecal administration. M is not a constant at all. It is a property of how an article would be given, and it has no value for a substance supplied for laboratory work, because no such assumption exists.

So an EU/mg figure is a measurement, not a verdict. It tells you the endotoxin burden per milligram of solid, which is genuinely useful for comparing lots of the same material or for deciding whether a preparation is suitable for a cell-based system. It becomes a pass or fail only once someone supplies the missing assumption, and a supplier of research material is not the party who can honestly supply it.

FDA's current guidance also notes that for a finished article with several components, the overall limit for parenterally administered products should not exceed the overall threshold limit specified in USP chapter <85>, regardless of any individual component's limit. That is a manufacturing constraint for approved articles, and it is a further reason that a per-milligram figure on a raw-material certificate is not a compliance statement about anything.

Why research certificates usually stop before it

There are three reasons.

First, relevance. A purity certificate answers what the material is and how much of it is the compound. Endotoxin answers a contamination question that matters for some downstream work, cell culture in particular, and not at all for others.

Second, cost. The assay is separate work with separate consumables, and one San Diego laboratory's public price page puts endotoxin by USP <85> in its own panel, priced above its identity and purity tiers. The published prices are compared in the tests a purity certificate does not include. Adding endotoxin to every lot is a cost carried by every vial.

Third, sample consumption. Each additional assay takes material out of a lot that was made to be sold, which for a small fill is not trivial.

None of that makes the assay unimportant. It makes its absence a fact to notice rather than a scandal: a certificate that reports purity, identity, water and net content has answered the questions it set out to answer, and has not answered this one.

What to read on a certificate that does carry it

The method: gel clot, turbidimetric, chromogenic, or a recombinant factor C assay.
The reported figure and its basis, EU/mg of solid, or EU per millilitre for a stated solution. The two are not interchangeable.
The sensitivity or detection limit of the assay as run, since "less than" is meaningless without it.
Whether inhibition and enhancement were assessed, which is how the laboratory establishes that the sample matrix is not suppressing or exaggerating the reading.
The laboratory, the report number and the date, as on any other assay.

How to read a certificate of analysis covers the rest of the page, and the tests a purity certificate does not include places endotoxin next to sterility, elemental impurities and residual solvents, which are routinely confused with it. The shorthand on any of these pages is defined in the peptide paperwork glossary.

The governing FDA document, as of September 21, 2026, is the March 2026 second edition of the pyrogen and endotoxins questions and answers guidance, and the compendial chapter it points to is USP <85>.

Sources

For laboratory research use only. Not a drug, not a supplement, and nothing here is a claim about what any of this material does in a person or an animal.

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