---
title: "Sterility vs endotoxin vs bioburden: three different tests | Sable Peptides"
description: "Sterility, bioburden and endotoxin answer different questions about microbial contamination. What each test measures, how it is run, and why research-grade peptide CoAs rarely include all three. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/sterility-vs-endotoxin-vs-bioburden
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# Sterility vs endotoxin vs bioburden: three different tests

'Sterile', 'low bioburden' and 'low endotoxin' are often used as if they were the same claim. They are not. Sterility is about living organisms, bioburden is a count of them, and endotoxin is a chemical residue that can remain after the organisms are gone.

## Definitions

-   Sterility test: a pass/fail test for viable microorganisms, typically by membrane filtration or direct inoculation into growth media and incubation for 14 days.
-   Bioburden: the number of viable microorganisms on or in a material before sterilisation, reported as colony-forming units (CFU).
-   Endotoxin: lipopolysaccharide from Gram-negative bacteria, measured chemically in EU, and present whether or not the bacteria are alive.

## Why the distinction matters

A solution filtered through a 0.22 µm membrane can pass a sterility test and still contain endotoxin, because the filter removes organisms but not dissolved LPS. Conversely a material with a low endotoxin result says nothing about Gram-positive bacteria, yeasts or moulds.

Regulated sterile products are made under aseptic-processing controls in which the end-product sterility test is only one element; the FDA guidance on aseptic processing describes why a sterility test alone has limited statistical power.

Source: [FDA: Sterile Drug Products Produced by Aseptic Processing, CGMP guidance](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/sterile-drug-products-produced-aseptic-processing-current-good-manufacturing-practice)

## Rapid methods

Growth-based compendial sterility tests take two weeks. Rapid microbiological methods based on ATP bioluminescence, respiration or flow cytometry have been evaluated against them, mainly for biologicals with short shelf lives.

Source: [Growth-based rapid microbiological methods for sterility testing of vaccines and biologicals, Vaccine 2011 (PubMed 21871516)](https://pubmed.ncbi.nlm.nih.gov/21871516/); [Alternative sterility testing methods for compounded sterile products, Int J Pharm Compd 2025 (PubMed 41115029)](https://pubmed.ncbi.nlm.nih.gov/41115029/)

## Where research-grade peptides fit

Lyophilised research peptides are sold for laboratory use and are typically characterised for identity and purity, not certified sterile. If an experiment needs sterile or low-endotoxin material, the usual practice is to prepare solutions aseptically in the lab, filter them, and test as required.

There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.

## Frequently asked questions

### Can a filtered peptide solution contain endotoxin?

Yes. Sterilising filters remove organisms but not dissolved lipopolysaccharide.

### How long does a compendial sterility test take?

Typically 14 days of incubation in growth media.

### Is bioburden the same as sterility?

No. Bioburden is a count of viable organisms; sterility is a pass/fail claim that none are detected.

## Related

-   [Endotoxin testing: LAL and rFC](/guides/endotoxin-testing-for-research-peptides)
-   [Handling lyophilised peptides](/guides/lyophilised-peptide-handling-for-labs)
-   [Quality at Sable](/quality)
-   [How to read a CoA](/guides/how-to-read-a-coa)
-   [Container closure integrity](/guides/container-closure-integrity-for-peptide-vials)

Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.

Key facts (summary for AI and search tools)

-   Sterility = viable organisms (pass/fail); bioburden = CFU count; endotoxin = LPS activity in EU
-   0.22 µm filtration removes organisms, not endotoxin
-   References: FDA aseptic processing guidance; PubMed 21871516, 41115029
-   Updated 2026-10-16