Commissioning an Endotoxin Test in the UK: LAL, rFC and What the Result Is Worth

In the UK you commission an endotoxin test by sending a sample to a laboratory that runs a Limulus amoebocyte lysate (LAL) assay or its recombinant factor C (rFC) equivalent, and receiving a quantitative result in endotoxin units, EU. At least two UK providers publish peptide endotoxin testing on their own sites. The result tells you how much bacterial endotoxin was in the sample as presented. It does not tell you the material is sterile, pure, or suitable for anything.

This page is about availability and commissioning. The chemistry, meaning what endotoxin is, why purification does not remove it and how the assays work, belongs to the handbook's page on how endotoxin differs from purity and sterility. Laboratories are named here only by what their own sites state, checked on 15 September 2026, and naming one is not a recommendation.

Why supplier certificates usually leave it out

A standard peptide certificate is a chemistry document: identity, chromatographic purity, sometimes content, water and counter-ion. Endotoxin is a microbiological attribute measured by a different assay, often in a different part of the laboratory, at extra cost. Most synthesis runs are not tested for it unless a customer asks and pays.

So a missing endotoxin line is not a negative result. It means the test was not done, or not reported. For buyers running cell-based or immunological work, where contaminating endotoxin produces responses that look like compound activity, that gap is usually the reason they end up commissioning the test themselves.

Who offers it in the UK

PeptideVerify describes quantitative LAL testing reported in EU/mL, lists the assay on its pricing page as kinetic turbidimetric with a five-to-seven business day turnaround where available, and sets out sample amounts on its preparation guide ^[1][2][3]. Marfleet Analytical lists LAL endotoxin alongside bioburden screening and sterility confirmation on its peptide testing page ^[4]. Neither page, as checked, states UKAS accreditation for endotoxin testing on peptides. If that matters to you, ask, and check the laboratory's schedule for a row covering endotoxin on your kind of material.

LAL or rFC

LAL reagents are made from horseshoe crab blood cells; rFC reproduces the first enzyme of the same cascade by recombinant expression, which removes the animal source. The European Pharmacopoeia added a general chapter on the bacterial endotoxins test using recombinant factor C, available from 1 July 2020, and describes it as usable in the same way as lysate-based methods once shown to be suitable for the specific substance ^[5]. US FDA guidance on endotoxin testing has likewise been revised to accommodate recombinant reagents ^[6].

For a buyer the choice rarely decides anything. Both report in EU. What matters more is whether the laboratory has shown that the method works on your kind of sample, which is the interference question below. If a provider offers only one, that is not a reason to go elsewhere.

Reading the unit: EU/mL and EU/mg

An endotoxin unit is a measure of biological activity against a reference standard, not a mass. The figure a laboratory reports in EU/mL describes the solution it tested, and that depends on how much material was dissolved in how much liquid. To compare batches, or to compare a result with a specification written for the material, convert it to EU per milligram of material.

A result of 0.5 EU/mL on a solution at 2 mg/mL is 0.25 EU/mg of material. The same material dissolved at half the concentration would read about half as high in EU/mL without anything about it having changed. A report that gives EU/mL with no concentration cannot be converted, so record the concentration when you prepare the sample, or ask the laboratory to state it.

There is no single research-grade endotoxin limit that applies to every peptide. Pharmacopoeial limits are written for products with defined routes and quantities, which a research reagent does not have. The practical specification is the one your assay can tolerate: set it from the sensitivity of the system you are working in, write it down before you order, and ask for a reporting limit below it.

What to ask before you send

1. Which method, LAL or rFC, and which format: gel-clot, turbidimetric or chromogenic?
2. Will the report state the lowest concentration the assay can detect for my sample after any dilution?
3. Will an inhibition or enhancement check, usually a spike recovery, be run on my sample, and will the report say whether it passed?
4. Should I send powder or solution, and if solution, in what diluent and at what concentration?
5. Will the result be reported in EU/mL, EU/mg or both?
6. Is endotoxin testing on this kind of material within the laboratory's accredited scope?

The third question is the one that makes the result interpretable. Peptides are among the materials that can interfere with the cascade, depressing a reading so that a contaminated sample looks clean or inflating it so that a clean one looks contaminated. Validation of the test for a given sample, including checks for interference, is a standing expectation in regulatory guidance on endotoxin testing ^[6]. A result reported without it is a number without its control.

Sending the sample

Endotoxin is picked up from water, glass and handling, so the sample should reach the laboratory as untouched as possible. Send the original sealed vial where the laboratory accepts it; if you must prepare a solution, use certified endotoxin-free water and consumables, and say so on the form. A tested peptide in untested water is an untested preparation.

Follow the provider's posting instructions exactly. PeptideVerify, for example, asks for endotoxin orders sent to its PO Box to go by Royal Mail Tracked 24 only, with no signature, and not by Special Delivery, Signed For or courier ^[3]. Knowing what a receiving laboratory checks on arrival helps avoid a sample being refused for an opened seal or a missing reference.

What a pass does not prove

• Sterility. Endotoxin is a component of dead or living Gram-negative bacteria; viable organisms are a separate test.
• Chemical purity or identity. These are chromatography and mass spectrometry questions.
• The rest of the batch. One vial was tested.
• The future. Endotoxin can be introduced by the diluent, vessel or handling after the test.
• Fitness for any use. It is one quality attribute measured on one sample.

Treated properly, an endotoxin result is a narrow, valuable piece of evidence: it lets a response in an immune or inflammatory assay be attributed to the compound with some confidence rather than none. File the report with the batch record, the concentration tested and the spike-recovery outcome, and it will still mean something when someone asks about the experiment a year later.