Endotoxin testing for peptides: what LAL and rFC measure
Some certificates for peptide material carry an endotoxin line. It usually refers to one of two assay families: LAL, made from horseshoe crab blood cells, or rFC, a recombinant form of one of the crab’s enzymes. Below we describe what endotoxin is, how each assay detects it and what the units mean. We also explain how labs check for interference, and what a low figure does not tell you.
Both assays detect bacterial endotoxin. LAL uses a lysate of horseshoe crab blood cells that clots or changes signal when endotoxin is present. rFC uses a recombinant crab enzyme, factor C, that endotoxin activates to cleave a fluorogenic substrate. Both report endotoxin units (EU). Neither is a test for other contaminants; for some other kinds, FDA says a different test may be more appropriate.
What endotoxin is
Endotoxin is lipopolysaccharide (LPS) from Gram-negative bacteria. A 2026 article puts it this way: “endotoxins are LPSs anchored in the outer layer of the bacterial outer membrane.” When the bacteria die and their cell walls break, LPS is released.[4] It is hard to destroy. The same article describes endotoxins as heat-stable and resistant to acid and alkali, with no structural damage even during distillation or standard sterilization.[4]
The units: EU, EU/mL and EU/mg
Endotoxin is not reported as a plain mass. “The quantification of endotoxins in terms of mass units (e.g., ng/mL) has been replaced by the use of endotoxin units (EU/mL),” a change dating from the late 1980s.[4] The same source gives a rough conversion of about 1 ng/mL to 5 to 10 EU/mL, which depends on the endotoxin preparation.[4]
A figure in EU/mg is a normalisation: the activity found, divided by the mass of sample tested. One 2025 study reported its figures this way, “normalized with respect to the nanoparticles’ weight.”[3] Read the unit before comparing two reports. A figure per millilitre of a test solution and a figure per milligram of powder are not the same thing.
This page gives no limit for research material. FDA’s endotoxin guidance is written for drug, biological product and device firms, not for research powders.[1]
How the LAL assay works
LAL stands for Limulus amebocyte lysate. The assay is “based on clotting of horseshoe crab blood in response to LPS.”[4] Three formats are in use, and “three types of LAL tests (gel-clot, chromogenic, and turbidimetric techniques) were harmonized in 2012” across the United States, Japanese and European pharmacopeias.[5] FDA’s guidance refers to the principles of the gel clot, photometric and kinetic methods.[1]
The formats differ in what they give you. Gel clot is run as a limit test: a clot forms or it does not. Chromogenic and turbidimetric formats read a color or a cloudiness and can give a number. FDA notes that “quantitative testing may be preferable to limit testing” when the aim is to follow trends over time.[1]
LAL has known weak points. The 2026 article says it is “sensitive to protease interference” and to β-glucan contamination.[4] The lysate contains factor G, a protein that β-glucan can act on without any endotoxin present, which is a route to a false positive.[2]
How the rFC assay works
Factor C is the crab enzyme that endotoxin activates. Its gene was cloned and expressed in other hosts, so the enzyme can be made without bleeding crabs. In the rFC assay, endotoxin activates recombinant factor C, which then cleaves a substrate to give a fluorescent signal.[2] Because “the rFC assay does not contain the factor G protein,” β-glucan cannot set it off.[2]
A 2024 study compared the two on reference endotoxin and on biopharmaceutical samples. On the reference material, “the rFC assay was as accurate as the LAL assay.” On real samples, “greater interference occurred in some samples when the rFC assay was used.”[2] Neither assay is free of interference.
| Format | What reacts | Read-out | Known weak points |
|---|---|---|---|
| LAL gel clot | Horseshoe crab blood-cell lysate | Clot or no clot (a limit test) | Proteases; β-glucan via factor G[2][4] |
| LAL chromogenic | Lysate plus a chromogenic substrate | Color, as a number | As above |
| LAL turbidimetric | Lysate | Cloudiness, as a number | As above |
| rFC | Recombinant factor C plus a fluorogenic substrate | Fluorescence, as a number | No factor G; more interference than LAL in some samples[2] |
Interference, and how labs check for it
A sample can push a reading down or up. FDA’s guidance says “bacterial endotoxins assays are subject to a variety of interferences related to the physical and chemical properties of the test article.”[1] The check for this is a spike. The lab adds a known amount of endotoxin to the sample and measures what share it gets back.
The 2025 study describes these “inhibition-enhancement controls (IECs)”: a known 0.5 EU/mL of endotoxin spiked into each test sample. A plate counted as valid only with “spike-recovery between 50 and 200%,” along with a good standard curve and a clean negative control.[3] Low recovery points to inhibition and high recovery to enhancement, which is what the control’s name refers to.
FDA’s guidance names lowering the sample concentration, within a set maximum, as one way to overcome interference.[1] In the 2024 study, rFC interference went away when the samples were tested that way.[2] No primary study was found that tests interference by research peptides as a class, so this page makes no claim about peptides in general.
On a report, look for the spike-recovery figure or a statement that the controls passed.
What a low reading does not tell you
An endotoxin assay measures endotoxin activity in the sample as tested. Nothing else. FDA’s guidance asks device and drug firms to assess, separately, the risk from other kinds of contaminant that endotoxin assays do not detect, and says a different test, the rabbit test, may be more appropriate where those may be present.[1] A low endotoxin figure is not a statement about other contaminants. For identity, purity and content, see How to read a certificate of analysis and Red flags on a certificate of analysis.
Questions people ask
What is endotoxin and where does it come from?
It is lipopolysaccharide from the outer membrane of Gram-negative bacteria, released when the bacteria die and break apart. It survives heat, acid and alkali.[4]
What is the difference between a LAL test and an rFC test?
LAL uses a lysate of horseshoe crab blood cells. rFC uses one recombinant crab enzyme, factor C, with a fluorescent read-out.[2][4] rFC lacks the factor G pathway, so β-glucan cannot trigger it. In one study it matched LAL accuracy on reference endotoxin but showed more interference in some samples.[2]
What does EU/mL or EU/mg mean on a lab report?
EU is an endotoxin unit, an activity unit that replaced mass units in the late 1980s.[4] EU/mL is per millilitre of the solution tested. EU/mg divides the activity by the mass of sample, a normalisation used in some studies.[3]
Do gel-clot, chromogenic and turbidimetric LAL tests give different kinds of readings?
Yes. Gel clot gives a pass or fail at a set sensitivity; the photometric and kinetic methods give a number. All three were harmonized across three pharmacopeias in 2012.[5] FDA notes that quantitative testing may be preferable to limit testing for following trends.[1]
Can a sample interfere with an endotoxin test, and how do labs check for it?
Yes. FDA describes a variety of interferences tied to the sample’s properties.[1] Labs spike a known amount of endotoxin into the sample and require recovery inside a set window, 50 to 200% in one published method.[3]
Does a low endotoxin reading mean a sample has no other contaminants?
No. Neither assay is a test for other contaminants. For some other kinds, FDA asks firms to assess the risk separately, and says a different test, the rabbit test, may be more appropriate where they may be present.[1]
Sources (fetched October 13, 2026)
- FDA, Endotoxins Testing: Questions and Answers (Edition 2), page dated 03/18/2026. https://www.fda.gov/media/83477/download (fetched October 13, 2026)
- “A Study on the Application of Recombinant Factor C (rFC) Assay Using Biopharmaceuticals.” Microorganisms. 2024;12(3):516. https://pmc.ncbi.nlm.nih.gov/articles/PMC10972375/ (fetched October 13, 2026)
- “Detection of Microbial Contamination in Nanomaterials Using LAL, rFC and Cell-Based Assays.” Nanomaterials (Basel). 2025;15(24):1871. https://pmc.ncbi.nlm.nih.gov/articles/PMC12736212/ (fetched October 13, 2026)
- Ishtiaq U, et al. Public Health Chall. 2026;5(4):e70397 (an overview of endotoxins). https://pmc.ncbi.nlm.nih.gov/articles/PMC13638776/ (fetched October 13, 2026)
- “Outstanding Contributions of LAL Technology to Pharmaceutical and Medical Science.” Biomedicines. 2021;9(5):536. https://pmc.ncbi.nlm.nih.gov/articles/PMC8150811/ (fetched October 13, 2026)