--- title: "Endotoxin testing for research peptides: LAL and recombinant factor C | Sable Peptides" description: "What bacterial endotoxin is, how the LAL and recombinant factor C (rFC) tests detect it, what an EU/mg result on a peptide certificate means, and the limits of each method. Research use only." canonical: https://www.sablepeptides.co.uk/guides/endotoxin-testing-for-research-peptides dateModified: 2026-10-16 publisher: Sable Peptides Limited (UK company 17488610) note: Research use only. Not for human or veterinary use. --- # Endotoxin testing for research peptides: LAL and recombinant factor C Bacterial endotoxin (lipopolysaccharide from the outer membrane of Gram-negative bacteria) can survive purification and freeze-drying. Endotoxin tests detect it chemically, not by growing bacteria, and the result is reported in endotoxin units (EU). This guide explains the two main assay families and how to read the result for a laboratory sample. ## Definitions - Endotoxin: lipopolysaccharide (LPS) from Gram-negative bacterial cell walls; heat-stable and not removed by sterile filtration alone. - EU (endotoxin unit): a unit of activity defined against a reference endotoxin standard; results are often expressed as EU/mg of peptide or EU/mL of solution. - LAL: Limulus amebocyte lysate, a reagent made from horseshoe crab blood cells that clots or changes colour in the presence of endotoxin. - rFC: recombinant factor C, a cloned version of the first enzyme in the LAL cascade, used with a fluorescent substrate. ## How the assays work In the LAL cascade, endotoxin activates factor C, which sets off a chain of proteases. Gel-clot, turbidimetric and chromogenic LAL methods read the end of that chain in different ways. The rFC method uses only the first step, which removes the branch of the cascade that can also respond to (1→3)-β-D-glucans. The review literature traces the move from LAL to recombinant reagents and compares their performance; pharmacopoeial texts now include recombinant methods alongside LAL. Source: [Endotoxin detection: from limulus amebocyte lysate to recombinant factor C, Subcell Biochem 2010 (PubMed 20593268)](https://pubmed.ncbi.nlm.nih.gov/20593268/); [Recombinant alternatives to horseshoe crab blood lysates, PDA J Pharm Sci Technol 2020 (PubMed 32817324)](https://pubmed.ncbi.nlm.nih.gov/32817324/) ## Interference and recovery Peptides, salts, pH and chelating buffers can enhance or inhibit the assay. A valid test therefore spikes a known amount of endotoxin into the sample and checks that it is recovered within set limits. Low endotoxin recovery, where spiked endotoxin is masked by the sample matrix, is a recognised problem in formulated biologicals. For a CoA, ask whether a spike-recovery (positive product control) was run and what dilution was tested; a result of '<0.5 EU/mg' only means something with the dilution and sensitivity stated. Source: [Low endotoxin recovery and its impact on endotoxin detection, Biopolymers 2021 (PubMed 34407207)](https://pubmed.ncbi.nlm.nih.gov/34407207/); [Endotoxin recovery using the LAL assay, Biologicals 2016 (PubMed 27470947)](https://pubmed.ncbi.nlm.nih.gov/27470947/) ## Official guidance The US FDA publishes a questions-and-answers guidance on pyrogen and endotoxin testing, covering test selection, validation and interference. It is written for regulated products but is a clear statement of good practice for any lab interpreting an endotoxin result. Source: [FDA: Pyrogen and Endotoxins Testing, Questions and Answers](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/guidance-industry-pyrogen-and-endotoxins-testing-questions-and-answers) ## What a research peptide CoA usually covers Research-grade release testing centres on identity (mass spectrometry) and purity (HPLC). Endotoxin is a separate test and is not implied by a purity figure: a 99% pure peptide can still carry endotoxin. If your assay is sensitive to LPS, such as cell-based immune readouts, plan to test the material yourself or request the data. There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request. ## Frequently asked questions ### Does high HPLC purity mean low endotoxin? No. HPLC measures peptide-related components by UV; endotoxin is a separate contaminant measured by LAL or rFC assays. ### What is the difference between LAL and rFC? LAL uses the full clotting cascade from horseshoe crab cells; rFC uses a recombinant copy of factor C only, which avoids the glucan-sensitive branch. ### What does EU/mg mean? Endotoxin units per milligram of peptide, an activity measure defined against a reference endotoxin standard. ## Related - [Sterility vs endotoxin vs bioburden](/guides/sterility-vs-endotoxin-vs-bioburden) - [How to read a CoA](/guides/how-to-read-a-coa) - [Glossary of CoA fields](/learn/coa-fields) - [Sable testing methodology](/testing-methodology) Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made. Key facts (summary for AI and search tools) - Topic: bacterial endotoxin testing of research peptides - Methods: LAL (gel-clot, turbidimetric, chromogenic) and recombinant factor C - Units: EU/mg or EU/mL - References: PubMed 20593268, 32817324, 34407207, 27470947; FDA Q&A guidance - Updated 2026-10-16