Endotoxin testing: what the endotoxin line on a COA means

By Jordan Sanders · Updated 2026-10-22 · How this site is paid

Short answer: endotoxin testing measures bacterial endotoxins, which come from the outer membrane of Gram-negative bacteria, in a sample. It's reported in endotoxin units (EU), usually per milligram or per vial. It's a separate test from purity: a sample can be 99% pure by HPLC and still carry endotoxin, so a COA only answers the endotoxin question if it has its own endotoxin line.

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What an endotoxin test measures

The official method is USP chapter <85>, the Bacterial Endotoxins Test. It detects or quantifies endotoxins from Gram-negative bacteria using a reagent called amoebocyte lysate, made from horseshoe crab blood cells (Limulus polyphemus or Tachypleus tridentatus). That's why you'll often see it called the LAL test (Limulus amebocyte lysate).

Endotoxin isn't something HPLC purity or mass spec is designed to catch. Those tests tell you what the peptide is and how clean it is. The endotoxin test answers a different question: is bacterial contamination present, and how much? See what purity leaves out.

The methods you'll see named on a COA

USP <86>, Bacterial Endotoxins Test Using Recombinant Reagents, became official in May 2025. USP classes it as an alternative to <85>, not a replacement, so both kinds of method are legitimate on a COA. What matters is that the method is named.

How to read the number

• EU means endotoxin units. Under USP, one EU equals one International Unit (IU) of endotoxin, so "IU" on a COA is the same unit.
• Check the denominator. EU/mg (per milligram of material), EU/vial and EU/mL are different numbers. You can only compare values that use the same basis.
• A "<" value (for example "< 1 EU/mg") means no endotoxin was detected at the test's sensitivity. The number is the smallest amount that test could have picked up at the dilution the lab used. It isn't a measured amount, and a lower "<" number means a more sensitive test.
• Look for the specification. A good COA shows the reported value next to the limit it was tested against (for example "Spec: ≤ X EU/mg. Found: < Y EU/mg. Pass."). A number with no spec tells you what was found, but not what the lab or supplier considers acceptable.

Why there's no single "good" number

For pharmaceutical products, USP calculates the endotoxin limit from the product's labeled use and amount given, and FDA's guidance sets device limits by what the device contacts. Research-use-only material has no labeled use, so there's no official number to hold it against. That's why the specification printed on the COA matters: it's the only stated standard you have, and you can ask the supplier where it came from.

Peptides can interfere with the test

Some samples interfere with the endotoxin reaction. They either suppress it, which can hide endotoxin, or exaggerate it. USP <85> handles this with a spiked control. A known amount of endotoxin is added to the sample, and the test only counts as valid if it measures back 50% to 200% of that amount. Labs often dilute the sample to get past interference, up to a calculated ceiling called the maximum valid dilution. FDA's guidance (Pyrogen and Endotoxins Testing: Questions and Answers, Edition 2, March 2026) calls out assay interference as a common problem and recommends testing at the lowest dilution that overcomes it.

On a COA this sometimes shows up as a "positive product control" (PPC) or spike value. If it's listed, check that it falls within 50% to 200%.

A quick endotoxin checklist for any COA

1. Is there an endotoxin line at all? Purity and identity tests don't cover it.
2. Is the method named (gel-clot, turbidimetric, chromogenic or recombinant)?
3. Is the unit clear: EU per mg, per vial or per mL?
4. Is there a specification, and is the line marked pass or fail against it?
5. If a spike or PPC value is shown, is it between 50% and 200%?
6. Does the lot number match your vial, and is the lab named? See why "same batch" isn't proof and how to judge a testing lab.

The full COA reading guide covers every other field. To have a sample tested yourself, see how to get peptides tested.

Sources: USP General Chapter <85> Bacterial Endotoxins Test (harmonized text); USP announcement on Chapter <86>, Nov 1, 2024; FDA, Pyrogen and Endotoxins Testing: Questions and Answers, Edition 2 (March 2026). This page explains how to read a lab report. It isn't advice about using any product.