What to Check When Planning Bacterial Endotoxins Testing

What the Endotoxins Test Detects

The Bacterial Endotoxins Test (BET) detects or quantifies endotoxins from gram-negative bacteria.[1][9] Traditionally, it uses a lysate of horseshoe crab blood cells (LAL, limulus amoebocyte lysate) as the reagent.[1][9]

Three Techniques and How Results Are Decided

• Gel-clot: the result is read from whether the reagent forms a gel.[1][2]
• Turbidimetric and chromogenic: photometric methods, each divided into end-point and kinetic techniques.[1][2]

Under ICH Q4B Annex 14, the endotoxins tests of the United States Pharmacopeia (USP <85>), the European Pharmacopoeia (Ph. Eur. 2.6.14) and the Japanese Pharmacopoeia (JP 4.01) can be used interchangeably in the ICH regions, subject to its conditions.[2][3] Any of the three techniques may be used, but in case of doubt or dispute the final decision is made with the gel-clot method.[1][3]

Endotoxin Limit and Maximum Valid Dilution (MVD)

For parenteral products, the endotoxin limit is calculated from the dose as K/M. M is the maximum dose per kilogram of body weight, and K is 5 EU/kg for routes other than intrathecal and 0.2 EU/kg for intrathecal administration.[1][4]

The maximum valid dilution (MVD) is the largest dilution at which the endotoxin limit can still be determined, calculated as "endotoxin limit × sample concentration ÷ reagent sensitivity (λ)".[1][5] When diluting a sample to overcome interference, the MVD must not be exceeded, because beyond it a sample above the limit could pass.[1][5]

Test for Interfering Factors (Inhibition and Enhancement)

It must first be confirmed that sample components do not inhibit or enhance the reagent reaction. In photometric methods, the measured value of endotoxin spiked into the sample must fall within 50–200% of the amount added.[1][5] This check is repeated when the reagent source, the manufacturing method or the formulation changes.[1][18] When changing reagents or testing laboratories, a common approach is comparative testing of both methods on samples from the same lot under a protocol approved in advance.[18][14]

A Caution for Protein Products: Low Endotoxin Recovery (LER)

In biologic formulations that contain both a chelating agent (e.g., citrate) and a surfactant (e.g., polysorbate), "low endotoxin recovery (LER)" has been reported, where spiked endotoxin becomes harder to detect as holding time passes.[15][16][17] PDA Technical Report No. 82 (2019) covers the mechanisms of LER and guidance on designing hold-time studies.[15][17] For such formulations, it is therefore worth checking separately whether endotoxin is recovered under the actual storage conditions and times, even if the test for interfering factors was passed.[15][16]

New Reagents and Changed Requirements

• European Pharmacopoeia: chapter 2.6.32, a test using recombinant factor C (rFC), was published in Supplement 10.3 and has applied since 1 January 2021. The method can be used like LAL-based methods once its suitability for the substance or product to be tested has been shown.[9][10]
• United States Pharmacopeia: chapter <86>, which uses recombinant reagents (rFC and recombinant cascade reagents), was approved in July 2024 and became official in May 2025. It does not replace <85> but adds an option, and users of LAL can continue as before.[6][7][8]
• The European Pharmacopoeia removed the rabbit pyrogen test from its monographs and added a new general chapter, 5.1.13 (Pyrogenicity). This applies from 1 July 2025, and manufacturers select an in vitro test, such as the monocyte activation test, based on a risk assessment.[11][12]
• The U.S. FDA issued Edition 2 (final) of "Pyrogen and Endotoxins Testing: Questions and Answers" in March 2026. As it revises the first edition of 2012, documents for U.S. submissions should be rechecked against Edition 2.[13][14]

Test Planning Checklist

Sources