Endotoxin testing checks a sample for fragments of bacterial cell walls that can trigger fever and inflammation, while sterility testing checks whether any living microorganisms are present. They answer different questions, and a sample can pass one and fail the other. Neither test says anything about whether a peptide is the right molecule or how pure it is. That is why these two tests sit alongside, not in place of, identity and purity testing.
This guide explains what each test measures, how the main methods work, what the results mean, and what they cannot tell you.
Key Takeaways
- Endotoxins are molecules (lipopolysaccharides) from the outer membrane of Gram-negative bacteria. They can cause fever even when the bacteria are dead [1][2].
- Sterility means no living microorganisms. A sterile product can still contain endotoxin.
- The main pharmacopoeial endotoxin tests (USP, Ph. Eur., JP) are recognised as interchangeable in ICH regions, under set conditions [3].
- Recombinant, animal-free endotoxin reagents are now in the European and United States pharmacopoeias [4][5].
- A high HPLC purity result does not mean a sample is low in endotoxin or sterile [6].
What Are Endotoxins?
Endotoxins, also called lipopolysaccharides (LPS), are large molecules that form part of the outer membrane of Gram-negative bacteria. When these bacteria die or break apart, endotoxins are released [2]. They are a classic example of a pyrogen, which simply means a substance that can cause fever.
Endotoxins matter in laboratory science for two reasons:
- Safety of injectable products. Even small amounts in an injected product can cause fever and, at higher exposures, serious reactions such as endotoxin shock [1].
- Reliability of experiments. Endotoxin contamination can confuse the results of cell and tissue studies, because cells may react to the contaminant rather than to the peptide being studied [1].
Endotoxins are also stubborn. Killing the bacteria does not remove them, and they can stick to other molecules during manufacturing. Contamination can come from water, raw materials, equipment and containers [2].
How Endotoxin Testing Works
The LAL test (horseshoe crab blood)
For decades, the standard bacterial endotoxins test (BET) has used Limulus amebocyte lysate (LAL), an extract of blood cells from horseshoe crabs. The lysate contains a clotting cascade that is set off by endotoxin. This largely replaced the older rabbit pyrogen test, where rabbits were injected and monitored for fever [4].
The pharmacopoeial chapters describe gel-clot and photometric techniques [3][8]. In plain terms:
| Technique | What the lab looks for | Output |
|---|---|---|
| Gel-clot | Whether a firm gel forms | Pass/fail against a limit |
| Turbidimetric | Increasing cloudiness | Quantitative (number) |
| Chromogenic | Colour change from a synthetic substrate | Quantitative (number) |
Under ICH Q4B Annex 14, the endotoxin chapters of the European Pharmacopoeia (2.6.14), the Japanese Pharmacopoeia (4.01) and the United States Pharmacopeia (<85>) are considered interchangeable in ICH regions, subject to conditions. If there is doubt or dispute, the gel-clot limit test is used to make the final decision [3].
Results are usually reported in endotoxin units (EU), for example EU per milligram or EU per millilitre. Pharmacopoeial reference standards are calibrated against the WHO International Standard for Endotoxin, so results can be compared across labs [3]. The acceptable limit is not a universal number. ICH notes that endotoxin limits should be set in the product's application dossier unless a monograph specifies them [3].
Recombinant factor C and other animal-free methods
Factor C is the protein in LAL that first detects endotoxin. Scientists have produced it recombinantly (made by engineered cells rather than harvested from crabs). Recombinant factor C (rFC) reagents have been available since the early 2000s, and published comparisons have found them equivalent and comparable to LAL [4].
Regulators and pharmacopoeias have followed:
- European Pharmacopoeia: Chapter 2.6.32, a test for bacterial endotoxins using recombinant factor C, became an official method in Supplement 10.3 (implemented January 2021) [7].
- United States: USP published Chapter <86>, Bacterial Endotoxins Test Using Recombinant Reagents, in May 2025. In March 2026 the FDA reissued its Pyrogen and Endotoxins Testing Q&A guidance as "Edition 2," removing certain LAL-specific references to accommodate recombinant reagents. The FDA adds that users should verify the method is suitable for their specific product [5][8].
Rules and pharmacopoeial texts are updated over time, so labs and readers should check the current version that applies in their jurisdiction.
Why interference matters
Peptides and other test materials can interfere with endotoxin assays, either hiding endotoxin or making it look higher than it is. That is why methods include suitability checks, such as spiking a known amount of endotoxin into the sample to confirm it can be recovered [3][5]. A good report will usually state the method and that suitability was demonstrated for that material.
What Is Sterility Testing?
Sterility testing looks for living bacteria and fungi. In the classic pharmacopoeial approach, samples are added to nutrient-rich liquid media, incubated for a defined period, and checked for signs of growth such as cloudiness (turbidity) [9].
There are two common set-ups:
- Membrane filtration: the sample is passed through a filter that traps microorganisms, and the filter is then incubated in growth media.
- Direct inoculation: a portion of the sample is placed straight into the growth media.
Under ICH Q4B Annex 8(R1), the sterility test chapters of Ph. Eur. (2.6.1), JP (4.06) and USP (<71>) are recognised as interchangeable in ICH regions, again under set conditions [10]. Faster "rapid microbiological methods" also exist and have been studied as alternatives to the traditional culture-based test [9].
The big limitation: sampling
A sterility test only examines a small number of units from a batch. If contamination is rare and scattered, a sample can pass by chance. For this reason, the European Union's GMP rules for sterile products say the finished-product sterility test should be regarded only as the last in a series of critical control measures. It cannot make up for a product that was not made under validated conditions [11]. In other words, sterility is built in by the manufacturing process; the test is a final check.
Endotoxin vs. Sterility: A Side-by-Side View
| Endotoxin testing | Sterility testing | |
|---|---|---|
| What it detects | Bacterial cell-wall fragments (LPS), living or dead | Living bacteria and fungi |
| Main concern | Fever and inflammatory reactions; confounded experiments | Infection risk |
| Typical output | A number in EU/mg or EU/mL, or pass/fail at a limit | Pass ("no growth") or fail |
| Can a sample pass this and fail the other? | Yes: a sterile sample can still carry endotoxin | Yes: a low-endotoxin sample can still carry living organisms such as Gram-positive bacteria or fungi |
Why a "Pure" Peptide Can Still Fail
It is easy to assume that a sample with a high purity figure on its Certificate of Analysis must be clean in every sense. It is not so. HPLC testing separates and measures peptide-related compounds; it is not designed to detect endotoxin or microorganisms. (See our guide to identity vs. purity vs. quantity testing for what each test type covers, and how to read a peptide lab report to find these results on a report.)
Real-world data illustrate this. In a 2024 study, researchers bought semaglutide products from illegal online sellers and analysed them. The freeze-dried samples contained no viable microorganisms at the time of testing, but endotoxin was detected in every sample, and measured purity was far below the figure claimed on the labels [6].
For quality-control purposes, the European Medicines Agency's 2025 guideline on synthetic peptides lists bacterial endotoxins and microbiological purity among the tests that may be included in a peptide active substance specification, alongside identity, purity and assay [12]. (This is an EU guideline for medicines, applicable since 1 June 2026; other jurisdictions set their own requirements.)
Regulatory Status
Endotoxin and sterility requirements are set by medicines regulators and pharmacopoeias, and they apply to products regulated as medicines in each jurisdiction. For example, the FDA sets expectations in the United States, while in the European Union the European Pharmacopoeia and EU GMP guidelines apply. Other regulators, such as the MHRA (UK), TGA (Australia) and Health Canada, apply their own frameworks. Regulatory status varies by jurisdiction and may change over time. Consult the relevant regulatory authority for current information.
Frequently Asked Questions
What is the difference between endotoxin testing and sterility testing?
Endotoxin testing measures bacterial cell-wall fragments that can cause fever, whether the bacteria are alive or dead. Sterility testing checks for living microorganisms. A sample can pass one test and fail the other.
What does "EU/mg" mean on a lab report?
It stands for endotoxin units per milligram. It is a measure of endotoxin activity relative to a reference standard calibrated against the WHO International Standard [3]. Whether a value is acceptable depends on the limit set for that specific product.
Is the LAL test still used?
Yes. LAL-based methods remain in the pharmacopoeias. Recombinant alternatives are now also recognised in the European Pharmacopoeia and USP [5][7].
Does filtering or freeze-drying remove endotoxin?
Not reliably. Endotoxins are released when bacteria die, can bind to other molecules, and are not removed simply by killing bacteria. Specialised removal steps exist, but they are part of controlled manufacturing [1][2].
If a sample passes a sterility test, is the whole batch sterile?
Not necessarily. The test samples only a small part of a batch, so sterility assurance depends mainly on validated manufacturing processes [11].
References
- Magalhães PO, Lopes AM, Mazzola PG, et al. Methods of endotoxin removal from biological preparations: a review. J Pharm Pharm Sci. 2007;10(3):388-404. https://pubmed.ncbi.nlm.nih.gov/17727802/ ↗
- Schneier M, Razdan S, Miller AM, Briceno ME, Barua S. Current technologies to endotoxin detection and removal for biopharmaceutical purification. Biotechnol Bioeng. 2020;117(8):2588-2609. https://doi.org/10.1002/bit.27362 ↗
- ICH. Q4B Annex 14: Bacterial Endotoxins Test General Chapter. https://database.ich.org/sites/default/files/Q4B%20Annex%2014%20Guideline.pdf ↗
- Tindall B, Demircioglu D, Uhlig T. Recombinant bacterial endotoxin testing: a proven solution. BioTechniques. 2021;70(5):290-300. https://doi.org/10.2144/btn-2020-0165 ↗
- U.S. Food and Drug Administration. FDA Clarifies Current Thinking on Pyrogen and Endotoxins Testing. https://www.fda.gov/science-research/advancing-alternative-methods-fda/fda-clarifies-current-thinking-pyrogen-and-endotoxins-testing ↗
- Ashraf AR, Mackey TK, Vida RG, et al. Multifactor quality and safety analysis of semaglutide products sold by online sellers without a prescription. J Med Internet Res. 2024;26:e65440. https://doi.org/10.2196/65440 ↗
- European Directorate for the Quality of Medicines & HealthCare (EDQM). Recombinant factor C: Ph. Eur. Chapter 2.6.32 (presentation, 2021). https://www.edqm.eu/documents/52006/115334/recombinant-factor-c-emmanuelle-charton.pdf/ea9de4ea-f831-d948-b834-986c32352533 ↗
- U.S. Food and Drug Administration. Pyrogen and Endotoxins Testing: Questions and Answers (Guidance for Industry, March 2026). https://www.fda.gov/regulatory-information/search-fda-guidance-documents/pyrogen-and-endotoxins-testing-questions-and-answers ↗
- Parveen S, Kaur S, David SA, et al. Evaluation of growth based rapid microbiological methods for sterility testing of vaccines and other biological products. Vaccine. 2011;29(45):8012-8023. https://doi.org/10.1016/j.vaccine.2011.08.055 ↗
- ICH. Q4B Annex 8(R1): Sterility Test General Chapter. https://database.ich.org/sites/default/files/Q4B%20Annex%208%28R1%29%20Guideline.pdf ↗
- European Commission. EudraLex Volume 4, Annex 1: Manufacture of Sterile Medicinal Products (2022). https://health.ec.europa.eu/medicinal-products/eudralex/eudralex-volume-4_en ↗
- European Medicines Agency. Guideline on the Development and Manufacture of Synthetic Peptides (EMA/CHMP/CVMP/QWP/367182/2025). https://www.ema.europa.eu/en/development-manufacture-synthetic-peptides-scientific-guideline ↗
This article is for educational purposes only and is not medical advice. It does not provide instructions for using any product. For health decisions, consult a qualified healthcare professional, and for regulatory questions, consult the medicines regulator in your jurisdiction.