Endotoxin testing and sterility testing are microbiological and contamination assays that answer different questions from the purity and identity numbers printed on a typical certificate of analysis (CoA). A standard research-peptide CoA documents composition - how pure the compound is and whether it is the molecule claimed. Endotoxin and sterility are separate tests, run under separate compendial standards, and their absence from a purity CoA is common. This guide explains what each test measures, why one result cannot stand in for the other, and how to read their presence or absence as a documentary fact rather than a guarantee. Research peptides are sold for research use only, and everything here concerns documentation, not use.
What does endotoxin testing actually measure?
Endotoxins are bacterial lipopolysaccharides - fragments of the outer membrane of certain bacteria. They are measured by the LAL (Limulus Amebocyte Lysate) assay, and results are reported as a concentration such as EU/mg or EU/mL, where EU stands for endotoxin units. The relevant compendial chapter is USP <85> (bacterial endotoxins test). An important point for reading a CoA: an endotoxin result describes the quantity of these specific molecular fragments detected in the sample, nothing more. It does not describe whether living organisms are present, nor does it describe chemical purity. Because the LAL assay targets one class of contaminant, it is a narrow, quantitative measurement. When a vendor publishes an endotoxin figure, note the unit and the batch it applies to; like every laboratory result, it characterises one lot at one point in time. To see how such evidence is weighted, review the PeptideTrust scoring methodology.
What does sterility testing actually measure?
Sterility testing detects the presence of viable microorganisms - living bacteria or fungi capable of growing under test conditions. The governing compendial chapter is USP <71> (sterility). Where the LAL assay counts a chemical marker, a sterility test asks a biological question: will anything grow when the sample is introduced to a suitable culture medium and incubated? A sterility result therefore speaks to viable contamination, not to chemical composition or to the endotoxin load. It is worth being precise about scope. Sterility testing is a separate procedure with its own methods, controls, and incubation periods, and it produces a pass or fail outcome for the batch tested. As with any batch-level result, it describes the specific lot analysed on the date of analysis. Treat a sterility statement the way you would treat a purity figure in the anatomy of a CoA: read the batch number, the date, and the named laboratory before you read the conclusion.
Why does passing one test not imply the other?
This is the single most important point, and it follows directly from what each test measures. Sterility looks for viable microorganisms; endotoxin looks for a chemical fragment of bacterial membrane. A sample can be sterile and still carry endotoxins, because endotoxins are molecular residues that persist after the organisms that produced them are gone. Conversely, a low endotoxin figure says nothing about whether living organisms are present. The two assays run under different chapters - USP <71> for sterility and USP <85> for endotoxin - precisely because they answer independent questions. For a reader assessing documentation, the practical rule is simple: passing one does not imply the other, and a CoA that reports one result has told you nothing about the other. Do not let a single reassuring line stand in for both. Each is a distinct measurement that must be documented on its own terms, with its own method, unit, and batch reference.
How do these tests differ from purity and identity on a CoA?
A conventional peptide CoA is built from two chemistry measurements. Purity comes from HPLC (high-performance liquid chromatography), which separates the sample into peaks and reports the target peak as a percentage of total peak area. Identity comes from mass spectrometry (MS or LC-MS), which measures molecular mass to confirm the compound is the peptide claimed. Endotoxin and sterility sit outside both. The table below summarises the distinct scope of each.
| Test | What it measures | Method / standard |
|---|---|---|
| Purity | Target peak as % of peak area | HPLC (chromatogram) |
| Identity | Molecular mass of the compound | MS / LC-MS |
| Endotoxin | Bacterial lipopolysaccharide load (EU/mg, EU/mL) | LAL, USP <85> |
| Sterility | Presence of viable microorganisms | USP <71> |
For the distinction between the two chemistry tests specifically, see HPLC versus mass spectrometry. The point here is that four different questions require four different measurements.
Why is endotoxin and sterility data usually absent from a purity CoA?
A standard research-peptide purity and identity CoA usually does not include sterility or endotoxin data, and that absence is common. These are additional assays, run under separate compendial chapters, often at additional cost and sometimes at a different laboratory. A CoA that reports HPLC purity and an MS identity confirmation has done what that document is designed to do: characterise chemical composition. The critical reading skill is what you conclude from the gap. The absence of endotoxin or sterility results is a documentary fact - it means those tests are not reported here - and it must not be read as a sterility guarantee or as evidence of contamination either way. Silence is silence. If a vendor does publish endotoxin or sterility figures, treat them as extra data points to verify against a named lab and a batch number, not as a reason to relax the checks you apply to every other line on the certificate.
What should you look for when this data is present?
When endotoxin or sterility results do appear, read them with the same scrutiny you apply to any laboratory claim. The following are the documentary signals worth checking:
- A named, independent laboratory, not an anonymous "tested externally" line - accreditation to ISO/IEC 17025 is a stronger signal of testing competence.
- A batch or lot number that matches the number printed on the vial, so the result maps to the material in hand.
- A date of analysis, because a result describes a batch at one point in time.
- The correct units and standard cited - EU/mg or EU/mL under USP <85> for endotoxin, USP <71> for sterility.
- A verification path that resolves on the laboratory's own domain rather than a PDF hosted only by the vendor.
These are the same transparency principles the scorecard applies across every category. To see how each becomes a weighted criterion, read the full methodology. Transparency is a documentary property, not a marketing claim.