Endotoxin Testing: LAL, rFC and EU/mg Explained (2026)

Endotoxin testing measures lipopolysaccharide (LPS), a molecule from the outer membrane of Gram-negative bacteria, in a sample. The standard methods use Limulus amebocyte lysate (LAL), a reagent made from horseshoe crab blood cells that clots or changes colour in the presence of LPS, or recombinant factor C (rFC), a cloned version of the enzyme that starts that reaction. Results are reported in endotoxin units (EU) per millilitre of test solution or per milligram of material, and the pharmacopeial method for LAL testing is USP <85>, Bacterial Endotoxins Test.

Endotoxin testing methods at a glance

Source: USP <85> method descriptions; Iwanaga [5]; Ding and Ho [8]. Sensitivity depends on the reagent lot and is stated on each reagent’s certificate.

What is endotoxin?

Endotoxin is the common name for bacterial lipopolysaccharide. An LPS molecule has three parts: a hydrophobic anchor called lipid A, a short core oligosaccharide, and a long, variable polysaccharide chain called the O-antigen [1]. Lipid A carries the biological activity that endotoxin tests detect, and it is often called endotoxin in its own right [1]. The O-antigen varies between strains, so “endotoxin” names a family of related molecules [1].

Three properties make endotoxin a persistent contaminant in laboratory materials:

• It is everywhere Gram-negative bacteria grow. Water, glassware, culture supplements and proteins made in E. coli are common sources. Endotoxin is a recognised contaminant of commercially available proteins and other biological preparations [11].
• It is stable. Reviews of pyrogen detection note that endotoxin is not eliminated by standard sterilisation processes [12]. A sterile solution can therefore still contain endotoxin. Labs remove it from glassware with dry heat at high temperature, a step called depyrogenation.
• It is amphiphilic. The fatty acyl chains of lipid A make LPS aggregate in water and stick to surfaces and to other molecules. That behaviour complicates both removal and measurement [11].

Why endotoxin matters in cell-culture and assay work

The reason to test a research reagent for endotoxin is data quality. Many mammalian cell types carry receptors that respond to very small amounts of LPS, so an endotoxin-contaminated reagent can produce a signal that belongs to the contaminant rather than to the molecule under study.

The receptor side: TLR4 and MD-2

Cells detect LPS through a receptor complex made of Toll-like receptor 4 (TLR4) and a partner protein, MD-2 [1]. The crystal structure of this complex shows LPS sitting in a large hydrophobic pocket in MD-2, with five of its six lipid chains buried inside the pocket and the sixth bridging to TLR4 [2]. Binding drives two receptor complexes together into an “m-shaped” multimer, which is the activated form [2].

What a contaminated reagent does to an experiment

A 2014 study measured endotoxin in commercially available recombinant proteins. Levels were generally in the range stated on data sheets, where most suppliers state below 1 EU, but some were higher [3]. The authors then exposed several human immune-cell types in culture to LPS at 0.002 to 2 ng/mL. Primary CD1c+ dendritic cells were activated by amounts equivalent to the contamination found in some of the commercial proteins, and the authors concluded that low-level contamination may generate erroneous data in LPS-sensitive cell systems [3].

Contamination does not only come from the reagent being studied. In one cell-culture laboratory, endotoxin above the lab’s limit appeared in both used and unused culture medium. Testing each ingredient separately traced it to one medium supplement, with all three lots of that supplement above the limit [4]. The authors’ recommendation was to measure endotoxin in reagents directly or to confirm it from an adequate certificate of analysis from the manufacturer [4].

For peptide work, the practical point is the same. If a peptide is going into a cell-based assay, especially one with macrophages, monocytes, dendritic cells or reporter lines built on TLR4 signalling, the endotoxin content of the peptide lot is part of the experiment’s controls.

How LAL endotoxin testing works

The LAL test grew out of the observation that blood cells from the horseshoe crab clot in response to Gram-negative endotoxin. Their lysate contains a cascade of serine protease zymogens: factor C, factor B and the proclotting enzyme [5]. Endotoxin activates factor C. Factor C activates factor B, factor B activates the proclotting enzyme, and the clotting enzyme converts the protein coagulogen into an insoluble coagulin gel [5]. Reconstructing the cascade from purified components, with endotoxin present, reproduces the gel [5].

The three LAL methods all use this cascade and differ in what they read:

• Gel-clot is the oldest format. A tube of lysate and sample is incubated and inverted. A firm gel that holds means endotoxin is at or above the lysate’s labelled sensitivity, written as lambda (λ) and commonly between 0.03 and 0.25 EU/mL. Running a dilution series gives a semi-quantitative endpoint.
• Kinetic turbidimetric reads the rising cloudiness of the reaction over time. More endotoxin means the solution reaches a set optical density sooner.
• Kinetic chromogenic replaces the clotting step with a synthetic peptide substrate. The activated enzyme cuts off p-nitroaniline, which is yellow, and the reader times how long it takes to reach a set absorbance at 405 nm.

Kinetic methods are read against a standard curve built from a reference endotoxin, so they return a number rather than a pass or fail. Before LAL, endotoxin was detected with an in vivo animal test. LAL entered use in the 1970s as an in vitro test, and an in vitro monocyte activation test followed in 2010 [12].

Glucans: the main source of false LAL signals

Horseshoe crab lysate has a second trigger. Factor G, also in the lysate, is activated by (1→3)-β-D-glucans and feeds into the same clotting enzyme [5]. Glucans come from fungi and from cellulose-based materials, including some filters. Commercial LAL reagents vary in how strongly they respond to glucans, which can produce conflicting results between laboratories [6]. Glucan-blocking buffers and kinetic methods are used to identify and suppress this signal [6].

Recombinant factor C: a non-animal endotoxin test

Recombinant factor C (rFC) takes the first enzyme of the cascade and produces it by genetic engineering. The recombinant protein is a proenzyme of about 132 kDa that is activated by trace levels of endotoxin [8]. Activated rFC cuts a fluorogenic substrate, and the assay reads fluorescence. Because the reagent contains no factor G, glucans do not trigger it.

Head-to-head comparisons report both agreement and differences:

• An rFC assay detected most LPS structures tested in picogram quantities. LPS from E. coli, Burkholderia cepacia and two Salmonella forms gave the same potency by rFC and by LAL, but LPS from several other species reacted differently between the assays [9].
• A validation study by a national regulatory laboratory found rFC as accurate as LAL when both were tested against reference standard endotoxin. Some biopharmaceutical samples interfered more with rFC than with LAL, and dilution overcame that interference [10].

The European Pharmacopoeia and the USP have both added chapters for recombinant reagents (Ph. Eur. 2.6.32 and USP <86>). A COA that cites one of these chapters used rFC rather than lysate.

EU/mL, EU/mg and the reference standard

Endotoxin is not reported as a mass because LPS from different bacteria differs in activity. Instead, activity is compared with a reference standard endotoxin. The US national reference was prepared from E. coli O113 by hot aqueous phenol extraction and characterised chemically and biologically [7]. One endotoxin unit is defined against that standard, and lysate lots are calibrated to it.

The unit on a report depends on what the lab tested:

• EU/mL describes the solution that went into the assay. It depends on how much material the lab dissolved and how far it diluted the sample.
• EU/mg describes the material itself. It is calculated by dividing the EU/mL result by the concentration of the test solution in mg/mL. A result of 0.05 EU/mL from a solution at 1 mg/mL is 0.05 EU/mg. The same 0.05 EU/mL from a solution at 0.1 mg/mL is 0.5 EU/mg.

That arithmetic is why an EU/mL figure with no stated test concentration cannot be converted to a per-milligram value.

Interference checks and the valid dilution

Peptides, salts, extreme pH and chelating agents can inhibit or enhance the LAL reaction. USP <85> therefore requires an interference test: a known amount of endotoxin is spiked into the sample, and the measured spike must fall within 50% to 200% of its expected value. If it does not, the sample is diluted and tested again. The chapter also defines a maximum valid dilution, the most a sample can be diluted while the method can still detect the endotoxin limit that applies to it. Some labs print the spike result next to the sample result.

How endotoxin testing appears on a peptide COA

Not every peptide COA includes endotoxin. When it does, the line should name the method and the unit. The worked examples below come from two lab reports published on the Vortex Research COA page.

Source: vortexresearch.net/coa/, checked October 21, 2026.

The MOTS-C report also notes that no universal pass/fail limit applies to research-use materials, so the lab reports a value rather than a pass. That is the correct way to read most research-reagent endotoxin results: as a measured value for the tested sample, which the researcher compares with the tolerance of their own assay.

When reading any endotoxin line, check four things:

1. Method. Gel-clot, kinetic turbidimetric, kinetic chromogenic or rFC.
2. Chapter. USP <85>, or a recombinant-reagent chapter for rFC.
3. Unit. EU/mL or EU/mg, and for EU/mL, the test concentration.
4. Sample. The lot or sample ID, matched to the vial. Our field-by-field guide to reading a peptide COA covers the other lines on the same report.

Endotoxin testing sits alongside the purity and identity tests, not in place of them. HPLC purity does not see LPS. Our explainers on HPLC peptide purity testing and how peptides are made cover where each kind of impurity comes from. For supplier-level checks, see how to evaluate a research peptide supplier.

Endotoxin versus sterility

The two tests answer different questions. Sterility testing asks whether living microorganisms are present. USP <71> does this by culture: samples are incubated in growth media for at least 14 days and inspected for growth. Endotoxin testing asks whether a bacterial cell-wall component is present, whether or not the bacteria are alive. A sample can pass sterility and still carry endotoxin from bacteria killed earlier in manufacture. A sample can also show low endotoxin while carrying Gram-positive organisms or fungi, which LAL does not detect.

Frequently asked questions

What is endotoxin?

Endotoxin is lipopolysaccharide (LPS) from the outer membrane of Gram-negative bacteria. It has a lipid A anchor, a core oligosaccharide and a variable O-antigen chain. Lipid A carries the activity that endotoxin tests detect.

What is the difference between LAL and rFC endotoxin testing?

LAL uses a lysate of horseshoe crab blood cells containing a whole clotting cascade. rFC uses only the first enzyme of that cascade, factor C, made by recombinant technology and read by fluorescence. rFC lacks factor G, so it does not respond to glucans.

What does EU/mg mean on a COA?

EU/mg is the endotoxin activity per milligram of the material tested. It equals the EU/mL result divided by the concentration of the test solution in mg/mL. An EU/mL result without a stated concentration cannot be converted to EU/mg.

Does a sterility result cover endotoxin?

No. Sterility testing looks for living microorganisms, while endotoxin testing measures a bacterial cell-wall molecule that remains after the bacteria are dead. They are separate tests and should appear as separate lines on a report.

Research materials: Vortex supplies research peptides as lyophilised reagents: browse the catalogue. Lab reports, including endotoxin results where tested: /coa/.

References

1. Raetz CR, Whitfield C. Lipopolysaccharide endotoxins. Annu Rev Biochem. 2002;71:635-700. PMID 12045108. doi:10.1146/annurev.biochem.71.110601.135414
2. Park BS, Song DH, Kim HM, Choi BS, Lee H, Lee JO. The structural basis of lipopolysaccharide recognition by the TLR4-MD-2 complex. Nature. 2009;458(7242):1191-5. PMID 19252480. doi:10.1038/nature07830
3. Schwarz H, Schmittner M, Duschl A, Horejs-Hoeck J. Residual endotoxin contaminations in recombinant proteins are sufficient to activate human CD1c+ dendritic cells. PLoS One. 2014;9(12):e113840. PMID 25478795. doi:10.1371/journal.pone.0113840
4. Uehara K, Oshiro E, Ochiai A, Takagi R, Yamato M, Kato A. Lessons learned from contamination with endotoxin originated from the supplement in the cell culture medium. Regen Ther. 2024;27:230-233. PMID 38596824. doi:10.1016/j.reth.2024.03.022
5. Iwanaga S. Biochemical principle of Limulus test for detecting bacterial endotoxins. Proc Jpn Acad Ser B Phys Biol Sci. 2007;83(4):110-9. PMID 24019589. doi:10.2183/pjab.83.110
6. Cooper JF, Weary ME, Jordan FT. The impact of non-endotoxin LAL-reactive materials on Limulus amebocyte lysate analyses. PDA J Pharm Sci Technol. 1997;51(1):2-6. PMID 9099058
7. Rudbach JA, Akiya FI, Elin RJ, et al. Preparation and properties of a national reference endotoxin. J Clin Microbiol. 1976;3(1):21-5. PMID 1254700. doi:10.1128/jcm.3.1.21-25.1976
8. Ding JL, Ho B. Endotoxin detection: from limulus amebocyte lysate to recombinant factor C. Subcell Biochem. 2010;53:187-208. PMID 20593268. doi:10.1007/978-90-481-9078-2_9
9. Abate W, Sattar AA, Liu J, Conway ME, Jackson SK. Evaluation of recombinant factor C assay for the detection of divergent lipopolysaccharide structural species and comparison with Limulus amebocyte lysate-based assays and a human monocyte activity assay. J Med Microbiol. 2017;66(7):888-897. PMID 28693666. doi:10.1099/jmm.0.000510
10. Kang DH, Yun SY, Eum S, et al. A study on the application of recombinant factor C (rFC) assay using biopharmaceuticals. Microorganisms. 2024;12(3):516. PMID 38543567. doi:10.3390/microorganisms12030516
11. Magalhães PO, Lopes AM, Mazzola PG, Rangel-Yagui C, Penna TC, Pessoa A Jr. Methods of endotoxin removal from biological preparations: a review. J Pharm Pharm Sci. 2007;10(3):388-404. PMID 17727802
12. Fennrich S, Hennig U, Toliashvili L, Schlensak C, Wendel HP, Stoppelkamp S. More than 70 years of pyrogen detection: current state and future perspectives. Altern Lab Anim. 2016;44(3):239-53. PMID 27494624. doi:10.1177/026119291604400305

Methods note: written by the Vortex Research Editorial Team. Literature was selected from PubMed for endotoxin chemistry and analytical methods, and every PMID was checked against its PubMed record for title, journal and year. USP and Ph. Eur. chapters are cited by name. COA data were read from vortexresearch.net/coa/ on October 21, 2026.

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