Endotoxin testing: what a buyer actually sees

An endotoxin result arrives as a number with a unit attached, and the unit is usually where the argument starts. Three assay families turn up on lab reports: LAL, rFC and MAT. They do not answer the same question. Two reports on one lot can look far apart and both be correct.

What each method is looking at

LAL uses a lysate from horseshoe crab blood. The lysate clots, or shifts colour, when it meets bacterial endotoxin. rFC swaps the lysate for a recombinant Factor C enzyme, so the readout is enzymatic from start to finish. MAT sits in a different family: it exposes a monocyte-based test system to the sample and reads the response, which can pick up pyrogens that are not endotoxin at all.

That difference sets the edge of each answer. A low LAL figure speaks about endotoxin and nothing else. It cannot clear a non-endotoxin pyrogen. MAT casts a wider net, but the readout is a biological response, so it travels less neatly between labs. None of the three inspects what your material actually is.

Reports rarely spell this out. A single line naming LAL or rFC looks like enough, and it is not. The format inside the family matters as much as the family itself, and the format is often the field that goes missing when a result is copied from one system to another.

The numbers and units on the page

• Result in EU/mL, or normalised to mass as EU/mg, or per container as EU/vial
• The dilution factor applied, plus the maximum valid dilution for the method
• Positive product control recovery, usually given as a percentage
• The standard curve range and the lowest point the assay can report
• Method name, reagent lot, analyst and the date the assay ran

A result printed as <0.05 EU/mL is a ceiling, not a measurement. The sample may hold a tenth of that figure or none of it, and the page will not say which. Read that line as a limit of reporting and nothing more.

Normalisation causes just as much trouble. One lab reports per millilitre of the solution it was handed. Another reports per milligram of powder. A third reports per vial. Line up those three without the basis and you are comparing nothing.

Why one lot gives different answers

Method choice comes first. A kinetic chromogenic LAL and a gel-clot LAL do not share a sensitivity, and an rFC kit can read the same sample differently again. Dilution comes second. The sample matrix interferes with the assay, and the dilution that clears the interference also pushes the result toward the floor of detection.

Sample handling adds spread on top. A different aliquot from the same lot, a longer hold before the assay, glassware from a different cabinet, one freeze-thaw cycle more than the other lab used. Two labs can run the same method on the same day and still part company, because one reports a ceiling and the other reports a number.

Questions worth putting to a supplier

• Which method, and which format inside that method
• What the unit is normalised to: volume, mass or container
• The dilution used, and whether recovery passed at that dilution
• Whether a below-detection result is a ceiling or a true reading
• Which aliquot of the lot was tested, and on what date

Put the questions in writing and keep the reply next to the report. A supplier who can name the method, the dilution and the recovery has given you something you can line up against the next lot. A supplier who forwards one figure with no basis has given you a number to admire.

It is also fair to ask whether the lab kept its raw data: plates, curves, worksheets. A summary page is enough for the file, but only if the underlying record still exists and can be requested. Without that, a figure has nowhere to go when a reviewer asks how it was produced.

A filed report needs a shelf. Ours is the record page: what arrives with each lot, in plain terms. The research-use page states the frame these materials are supplied under, and the cold-chain note covers the dispatch side of the same file.

Compare like with like

Two reports on one material only mean something when the method, the dilution basis and the unit all match. Check those three before you read a gap as a difference between lots. If they do not match, the gap belongs to the assays, not to the material. Note which of the two you are looking at, and the same question will not cost you an afternoon next quarter.

One habit helps more than any table. Record the report identifier and the date it arrived, not only the value. Values get quoted out of context in meetings. An identifier drags the context back with it.