Endotoxin and Sterility Testing Explained: LAL, rFC and USP 71 Results on a COA

Short answer: endotoxin testing and sterility testing are two different tests that answer two different questions. An endotoxin test (USP <85>, using Limulus amebocyte lysate (LAL), or USP <86>, using recombinant reagents such as recombinant Factor C (rFC)) measures bacterial endotoxin, a lipopolysaccharide from the outer membrane of Gram-negative bacteria. It reports the result in endotoxin units (EU) per mg or per mL. A sterility test (USP <71>) asks whether living microorganisms grow from the tested units over at least 14 days of incubation, and reports pass or fail. Neither is part of a standard identity-and-purity Certificate of Analysis. A result appears only when the test was ordered for that lot. Either way, a result is a measurement on tested samples of one lot by one method. It is not a statement about suitability for any purpose.

This guide explains how each method works and what the figures on a certificate mean. It is written for reading laboratory documentation.

Two tests, two questions

The two are independent. Endotoxin is a cell-wall molecule, not a living organism, so a material can be sterile and still carry endotoxin. A material with very low endotoxin can still contain viable organisms that the endotoxin test was never designed to see.

What endotoxin is, and why laboratories measure it

Endotoxin is the lipopolysaccharide (LPS) layer of the Gram-negative bacterial outer membrane. It is shed when bacteria grow and when they die, and it is heat-stable, so killing bacteria does not remove it. Water, glassware, reagents and process equipment are the usual sources.

For laboratory work, endotoxin is mainly a source of false results in cell-based experiments. Schwarz and colleagues measured the endotoxin content of commercially available recombinant proteins, then exposed primary human immune cells in culture to LPS at matching concentrations (0.002–2 ng/mL). They found that CD1c+ dendritic cells could be activated by those minimal amounts, and concluded that even low endotoxin contamination can generate erroneous data in sensitive cell systems¹. A peptide reagent carrying endotoxin can therefore produce a cellular response that has nothing to do with the peptide. That is why many cell-culture protocols specify an endotoxin limit for reagents. Counter-ions raise a similar concern, covered in salt forms and net peptide content.

How the endotoxin test works

Limulus amebocyte lysate (LAL): USP <85>

The test grew out of an observation by Levin and Bang that endotoxin triggers coagulation of horseshoe crab (Limulus) blood². A lysate of the crab’s amebocytes contains an enzyme cascade that begins with Factor C. When endotoxin activates Factor C, the cascade ends in a clot, turbidity or a coloured product, depending on the technique:

• Gel-clot: a limit test. A firm gel at the end of incubation means endotoxin is at or above the lysate’s labelled sensitivity (λ). It is the referee method when results disagree, unless a monograph says otherwise³.
• Turbidimetric (endpoint or kinetic): the rise in turbidity is read against a standard curve.
• Chromogenic (endpoint or kinetic): the cascade cleaves a synthetic substrate that releases a coloured product, read against a standard curve.

USP <85> has been harmonized with the European and Japanese Pharmacopoeias since the edition published in 2002⁴. FDA’s guidance on these tests, revised as Edition 2 in March 2026, addresses <85> and its gel-clot, photometric and kinetic methods³.

Recombinant Factor C (rFC) and recombinant cascade reagents: USP <86>

Recombinant reagents replace the crab-derived lysate with Factor C (or the full cascade) produced by expression systems. USP added General Chapter <86>, Bacterial Endotoxins Test Using Recombinant Reagents, in USP–NF 2025 Issue 1. It covers rFC and recombinant cascade reagent (rCR) methods as additional techniques alongside <85>^4,5. The chapter’s own status note describes its tests as alternatives unless an individual monograph specifies them⁵, and FDA guidance accepts alternative methods that are validated and shown to give equivalent or better results³.

One technical difference matters when reading results. LAL can also be triggered through a second pathway (Factor G, which responds to β-glucans), and rFC does not have that pathway⁶. On performance, Piehler and colleagues compared one kinetic chromogenic LAL assay with two rFC assays over six years of a proficiency-testing programme. Both rFC assays were comparable to LAL, met compendial acceptance criteria and gave better endotoxin recovery⁶. Marius and colleagues found an endpoint-fluorescence rFC assay less sensitive to interference than a kinetic LAL assay in purified water matrices⁷.

Reading an endotoxin result on a COA

Dilution and interference. The physical and chemical properties of a sample, including its pH, salts and the peptide itself, can suppress or enhance the reaction. Laboratories handle this by diluting the sample and by spiking a known amount of endotoxin into it, a positive product control, to show that the endotoxin can still be recovered. The most a sample may be diluted is the maximum valid dilution (MVD), defined from the endotoxin limit. If a test fails below the MVD, it is repeated at a greater dilution that does not exceed it, and FDA recommends screening at a dilution just above the point where interference disappears³. In practice, two certificates for the same material can carry different “<” values simply because they were tested at different dilutions.

Handling matters. FDA notes that storage and handling of the sample can change how much endotoxin is detected, and that purified endotoxin used as a spike may behave differently from native endotoxin³.

How the sterility test works: USP <71>

• Two ways to test. In membrane filtration, the sample is passed through a filter that retains microorganisms, and the filter goes into the media. In direct inoculation, the sample is added straight into the media.
• Two media. Fluid thioglycollate medium supports anaerobic as well as aerobic bacteria and is incubated at 30–35 °C. Soybean–casein digest medium supports fungi and aerobic bacteria and is incubated at 20–25 °C.
• Incubation of not less than 14 days, with the media examined for visible growth.
• Method suitability. The sample is first shown not to suppress the growth of challenge organisms, because a sample that inhibits growth could pass without being sterile.
• Number of units tested depends on the batch size, using tables in the chapter.

The chapter sets limits on what a pass means. A satisfactory result only shows that no contaminating microorganism was found in the sample examined under the conditions of the test, and the test is not by itself designed to ensure that a whole batch is sterile⁸. Sterility assurance comes from validating the sterilization or aseptic process, not from the end test. Portions of <71> are harmonized with the European and Japanese Pharmacopoeias⁸.

What these results do not tell you

• Not identity, purity or content. A clean endotoxin or sterility result says nothing about whether the peptide is the right sequence, how pure it is, or how much peptide the vial holds. Those come from identity and purity testing and from net peptide content.
• Not the other test. Sterile does not mean endotoxin-free, and low endotoxin does not mean sterile.
• Not the whole lot. Both are sample-based. The sterility test in particular can only show that no growth was found in the units tested.
• Not suitability for any use. A result shows that one lot met one specification by one method. It does not make a research material suitable for any use outside the laboratory.
• Not present unless ordered. A standard research-peptide COA reports HPLC purity and mass-spectrometry identity. If a certificate does not list an endotoxin or sterility result, those tests were not performed on that lot. A missing result is not a pass.

Where this fits in reading a COA

Identity and purity are the core of a peptide COA. See how to read a peptide COA, HPLC testing explained and mass spectrometry on COAs. Microbiological tests are additional panels, and like any result they are only as useful as the lot traceability behind them: what makes a COA easy to verify and what third-party testing means. Lot certificates for Peptide Titans products are in the certificate library. Glossary entries: endotoxin testing and sterility testing.

Frequently asked questions

References

1. 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
2. Levin J, Bang FB. The role of endotoxin in the extracellular coagulation of Limulus blood. Bull Johns Hopkins Hosp. 1964;115:265–274. PMID 14209047
3. US Food and Drug Administration. Guidance for Industry: Pyrogen and Endotoxins Testing: Questions and Answers (Edition 2). March 2026. FDA guidance page
4. United States Pharmacopeia. General Chapter <1085> Guidelines for Bacterial Endotoxins Testing, which describes the history of <85> Bacterial Endotoxins Test and the addition of <86>. USP–NF. doi.org/10.31003/USPNF_M2245_04_01
5. United States Pharmacopeia. General Chapter <86> Bacterial Endotoxins Test Using Recombinant Reagents. USP–NF. doi.org/10.31003/USPNF_M16015_02_01
6. Piehler M, Roeder R, Blessing S, Reich J. Comparison of LAL and rFC assays: participation in a proficiency test program between 2014 and 2019. Microorganisms. 2020;8(3):418. PMID 32188126
7. Marius M, Vacher F, Bonnevay T. Comparison of bacterial endotoxin testing methods in purified pharmaceutical water matrices. Biologicals. 2020;67:49–55. PMID 32753293
8. United States Pharmacopeia. General Chapter <71> Sterility Tests. USP–NF. doi.org/10.31003/USPNF_M98810_01_01

Research context: This guide explains analytical and microbiological test methods so laboratory documentation can be read accurately. A test result describes the tested samples of one lot. It does not establish suitability for any use outside qualified research.