Sterility testing and endotoxin testing answer different questions, and a passing result on one says nothing about the other. A sterility test determines whether viable microorganisms are present in a sample. The Bacterial Endotoxins Test (BET) determines whether lipopolysaccharide (LPS) from the cell walls of Gram-negative bacteria is present, and at what level. A product can be free of living organisms and still carry a pyrogenic load, because endotoxin survives the processes that kill the cells that produced it.
This article sets out the distinction in technical terms, explains the pharmacopoeial methods behind each test, covers the Australian regulatory context, and describes what laboratories should look for when reviewing documentation for research peptides and associated supplies. All products referred to in connection with Au Peptide Labs are supplied strictly for laboratory research, not for human or veterinary use.
The Short Comparison
| Parameter | Sterility test | Bacterial Endotoxins Test (BET) |
|---|---|---|
| Target | Viable, replicating microorganisms (bacteria, yeasts, moulds) | Endotoxin (LPS) from Gram-negative bacteria |
| Nature of the analyte | Living cells | Heat-stable cell-wall component, present whether or not cells are alive |
| Typical method | Membrane filtration or direct inoculation into culture media | Gel-clot, turbidimetric or chromogenic assay with lysate or recombinant reagent |
| Typical duration | Long incubation, often 14 days | Rapid, with gel-clot incubation of about 60 minutes |
| Pharmacopoeial chapter (USP) | 〈71〉 Sterility Tests | 〈85〉 Bacterial Endotoxins Test |
| Clinical concern it addresses | Infection from viable contaminants | Pyrogenic (febrile) reactions |
| Result is unit-based | No: pass or fail | Yes: Endotoxin Units (EU) per mL, mg or unit of product |
The two tests are complementary. The Therapeutic Goods Administration (TGA) states that sterile dosage forms must comply with the harmonised pharmacopoeial sterility test and, where applicable, the Bacterial Endotoxins Test.
What a Sterility Test Establishes
The WHO defines sterility as freedom from the presence of viable microorganisms, and notes that the conditions guaranteeing absolute sterility are usually too harsh for active ingredients. For that reason, sterility of a medicinal product is expressed in practice as a probability. The TGA guidance cited above describes sterility as a probability of less than 1 in 1,000,000 that a container is contaminated with replicating microorganisms. This is the concept of a sterility assurance level, which we discuss separately in Sterility Assurance Level for Research Peptides Explained.
Methods and their limits
The compendial sterility test in USP 〈71〉 uses either membrane filtration or direct inoculation of the sample into suitable culture media, followed by incubation and observation for growth. Incubation periods are long, commonly 14 days, because the test is designed to recover a broad range of organisms with differing growth rates.
Two limitations follow from this design:
- The test is destructive and statistical. Only a sample of units is tested, so a pass supports the conclusion that the batch was produced under a sterile process but does not prove every unit is sterile.
- It detects only what grows. Non-culturable organisms, dead cells and cell-wall fragments are invisible to it. Dead Gram-negative bacteria that leave LPS behind therefore pass a sterility test without difficulty.
For a stepwise view of what a laboratory should verify before releasing a sterile lot, see our 14 day peptide sterility checklist aligned to USP and WHO.
What the Bacterial Endotoxins Test Establishes
Endotoxins are lipopolysaccharides forming part of the outer membrane of Gram-negative bacteria. They are released when cells die or divide, and they are pyrogenic: in a clinical setting they provoke fever and, at higher exposures, more severe inflammatory responses. The TGA guidance describes BET as a control used to prevent pyrogenic reactions in patients.
Reagents and formats
The BET uses amoebocyte lysate derived from horseshoe crab blood, which clots or generates a measurable signal in the presence of endotoxin. According to USP 〈85〉, three technique families are recognised:
- Gel-clot. Endotoxin is detected by formation of a firm gel that remains intact when the tube is inverted 180° after incubation at 37 ± 1 °C for 60 ± 2 minutes. This format serves as the referee method in disputes.
- Turbidimetric. Turbidity development is measured photometrically, either as an endpoint or kinetically.
- Chromogenic. A chromophore is released by the enzymatic cascade and measured, again by endpoint or kinetic reading.
The same chapter specifies that one USP Endotoxin Unit equals one International Unit of endotoxin, and that the threshold pyrogenic dose (K) is 5 EU/kg for non-intrathecal administration and 0.2 EU/kg for intrathecal administration. These figures illustrate why results are reported in units: the question is never only whether endotoxin is present, but whether it is below a limit set for the product and route.
Recombinant alternatives
Recombinant Factor C (rFC) assays are increasingly accepted as alternatives to lysate from horseshoe crabs. They are valued for sustainability and for specificity, since they avoid the interference some lysates show with beta-glucans. As the FDA perspective presented through USP indicates, such methods require product-specific validation demonstrating they perform at least as well as the compendial approach, including accuracy, precision and inhibition or enhancement studies.
Interference and the Maximum Valid Dilution
A recurring practical problem is interference, where the sample itself inhibits or enhances the lysate reaction. The usual remedy is dilution, but dilution has a ceiling: the Maximum Valid Dilution (MVD). The MVD is not a fixed number. It is calculated from the product's endotoxin limit and the sensitivity of the reagent, and it represents the greatest dilution at which an endotoxin level at the limit would still be detected.
When the dilution needed to overcome interference exceeds the MVD, the sample cannot be validly tested by that method. The options are then a different reagent, sample pre-treatment, or specialised purification. For peptides, which are frequently formulated at high concentration or with excipients that affect enzymatic cascades, this is a genuine consideration rather than a theoretical one, and it is why a bare "endotoxin: pass" line carries less information than a report that states the method, the limit and the validation of the matrix.
Why a Sterile Product Can Still Fail BET
The central point is mechanistic. Sterilisation inactivates or removes organisms. Endotoxin is a chemical structure, not an organism, and it is exceptionally heat-stable. Three scenarios show the gap:
- Contaminated starting material. Water, buffers or excipients with a high Gram-negative bioburden can load endotoxin into a batch before any sterilising step.
- Terminal sterilisation after contamination. The organisms are killed, the sterility test passes, and the LPS remains.
- Contaminated primary packaging. Vials, stoppers or closures that were sterilised but not depyrogenated can transfer endotoxin to the product.
This is the reason the endotoxin question is separate from the sterility question, and the reason both appear in release specifications. The FDA regulation cited in the USP presentation (21 CFR 211.167(a)) requires, for each batch of drug product purporting to be sterile or pyrogen-free, appropriate laboratory testing to determine conformance with those requirements.
Does autoclaving remove endotoxin?
No. Moist heat at 121 °C kills vegetative bacteria and spores, but the LPS remains intact. Endotoxin inactivation requires high-temperature dry-heat processing, typically 250 °C for at least 30 minutes, or specialised filtration or removal techniques. The WHO GMP guidance for sterile pharmaceutical products is the reference document for these manufacturing controls. Treating an autoclave cycle as a route to a pyrogen-free article is a process failure, not a minor shortcut. The USP chapter itself refers readers to dry heat sterilisation for the validation of procedures that inactivate endotoxin.
The Australian Regulatory Context
In Australia, the microbiological quality framework for medicines is set through Therapeutic Goods Order No. 100 (TGO 100). Per the TGA guidance on meeting microbiological standards, TGO 100 specifies the minimum microbiological quality requirements a medicine should comply with throughout its shelf life, unless exempt.
The practical consequences for sterile medicines are as follows:
- Sterility and endotoxin testing are performed on each batch before release, unless the TGA has approved parametric release.
- Specifications for both tests belong in release and expiry protocols.
- Alternative rapid microbiological methods for either test require prior TGA review and validation demonstrating non-inferiority to the pharmacopoeial methods (USP 〈71〉 for sterility, 〈85〉 for endotoxin), covering accuracy, precision, specificity, limit of detection and robustness.
These requirements govern therapeutic goods. Research materials supplied for in vitro or other laboratory work are not therapeutic goods in the same sense, and their documentation expectations are set by the purchasing laboratory, its institution and any applicable approvals. Where a research programme moves toward use in humans in Australia, the sponsor's obligations change materially; the notification pathways are outlined in our TGA research exemptions checklist for Australian sponsors. Sponsors and investigators should confirm their position with the TGA and their institutional human research ethics committee.
Reading a Peptide Certificate of Analysis
Purity by HPLC and identity by mass spectrometry describe the peptide itself. Neither describes the microbiological state of the vial. A laboratory evaluating a supplier's documentation should therefore separate three layers of evidence.
Layer 1: identity and purity
A batch-specific certificate of analysis (COA) from an independent laboratory should state the batch number, the analytical method, the measured purity and the confirmed molecular mass. Batch traceability matters more than a headline percentage; the checks for confirming that a COA corresponds to the vial in hand are covered in 5 checks labs use to spot counterfeit peptides in Australia.
Layer 2: sterility evidence
If the intended application is sensitive to viable contamination (cell culture, for example), the laboratory should ask whether a compendial sterility test, or a validated equivalent, was performed on the lot, and what the sample size and incubation conditions were. A lyophilised peptide that has not been sterility tested should be handled and reconstituted with aseptic technique appropriate to the work.
Layer 3: endotoxin evidence
For endotoxin-sensitive assays (immune cell cultures, primary cells, in vivo animal work under ethics approval), the laboratory should request a quantitative result in EU per mg or per mL, with the method (gel-clot, kinetic turbidimetric, kinetic chromogenic or recombinant), the reagent sensitivity, and confirmation that interference was assessed within the MVD. An unqualified statement of "endotoxin free" is not a measurement.
A useful working checklist for incoming lots:
- Is the batch number on the COA identical to the label?
- Does the report name the testing laboratory and the date?
- Are purity and identity methods stated?
- Is there separate evidence for sterility and for endotoxin, or is one assumed from the other?
- Are endotoxin results quantitative, with units and a stated limit?
- Do the diluent and packaging components carry their own documentation?
Diluents, Syringes and Packaging as Contamination Routes
Even a well-characterised peptide can be compromised downstream. Reconstitution fluids, syringes and wipes each introduce sterility and endotoxin variables. The distinction between a multi-dose diluent containing a preservative and single-use sterile water bears directly on how long a reconstituted solution can be handled safely; we compare them in bacteriostatic vs sterile water. Note that a bacteriostatic agent inhibits growth of organisms; it does not remove endotoxin already present.
Primary packaging is a related issue, because closures and vials are potential carriers of both viable organisms and endotoxin. Our overview of peptide packaging materials for Australian laboratories addresses material selection. Syringes supplied for laboratory measurement and transfer are discussed in insulin syringes for pharmaceutical-grade lab research in Australia, and purchase rules differ by state, as set out in syringe purchase laws in Australia.
Our Perspective
We regard purity, sterility and endotoxin as three separate attributes that are frequently conflated in supplier marketing. A purity figure above 99% describes the proportion of the target peptide in the lyophilised material; it does not describe whether the vial contains viable organisms, and it does not describe pyrogen load. Laboratories are best served by asking for each attribute by name and by treating an unspecified claim as an unanswered question.
Our own position rests on what we can document. Our products are independently third-party tested to 99%+ purity, with verifiable COAs for every batch, and are dispatched from Melbourne so that domestic delivery is not subject to customs delay. We recommend that any laboratory with an endotoxin-sensitive or sterility-sensitive application state that requirement when ordering, so that the documentation supplied can be matched to the assay rather than assumed. For bulk or custom orders, requirements of this kind can be raised in advance.
We also hold that the interference problem is under-appreciated. A result generated without an assessment of inhibition or enhancement in the sample matrix can be falsely reassuring, and a result generated beyond the MVD is not valid at all. Researchers should treat the method statement on the report as part of the result.
— Dr. Authur
Sourcing Research Peptides in Australia
Laboratories that need batch-documented research peptides and associated consumables can browse our catalogue by research area: Metabolic & GLP-1, Recovery & Repair, and Lab Supplies, among others. Orders are dispatched from Australia within one business day, with discreet packaging, and Express Post is available for $10 (one-off) on the Au Peptide Labs store. All items are for laboratory research use only.
Frequently Asked Questions
Is a sterile product automatically free of endotoxin?
No. Sterility means no viable microorganisms, whereas endotoxin is a heat-stable cell-wall component that persists after the cells that produced it have been killed. A product made from contaminated materials and then sterilised can pass a sterility test and still fail the Bacterial Endotoxins Test.
Does autoclaving remove endotoxins?
No. An autoclave cycle at 121 °C kills vegetative bacteria and spores but leaves lipopolysaccharide intact. Depyrogenation generally requires dry heat, typically 250 °C for at least 30 minutes, or specific removal techniques such as validated filtration.
What is the Maximum Valid Dilution in the endotoxin test?
The MVD is the greatest dilution of a sample at which an endotoxin level equal to the product limit can still be detected. It is calculated from the product's endotoxin limit and the lysate sensitivity, so it differs for every product. If overcoming interference requires dilution beyond the MVD, the sample cannot be tested validly by that method.
Why does a sterility test take so long compared with BET?
Sterility testing relies on organism growth, so samples are incubated, often for 14 days, to recover slow-growing contaminants. The BET detects a biochemical reaction with lysate, and a gel-clot result is read after about 60 minutes of incubation. The speed of BET reflects the chemistry of the assay, not a lower evidentiary standard.
Sources
- TGA: Meeting microbiological standards for non-prescription and prescription medicines
- USP 〈85〉 Bacterial Endotoxins (harmonisation monograph)
- USP 〈85〉 Bacterial Endotoxins Test
- USP 〈71〉 Sterility Tests
- WHO: Sterility testing
- WHO TRS 1044 Annex 2: GMP for sterile pharmaceutical products