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Should I Test for endotoxins Pre- or Post-Sterilization?

Typically, end-product testing for endotoxin is performed post-sterilization. Performing testing post-sterilization allows for the entire manufacturing process to be evaluated for potential impact on the product or endotoxin levels. Although post-sterilization testing is typically recommended, pre-sterilization testing may be performed on certain products with proper justification and documentation.  

Before considering which products may be appropriate for pre-sterilization testing, the effect that sterilization may have on endotoxin recovery must be evaluated. Endotoxins are a highly stable components of the outer cell of Gram-negative bacteria, and while a recent review of industry data suggests that some endotoxins may be inactivated during sterilization, the level of endotoxin inactivation will vary depending on the sterilization modality and individual product. Also, as sterilization will kill live bacteria, it will not remove the bacteria from the surface of a product; therefore, at least some endotoxins that were present in the cell wall are likely to remain post-sterilization.   

While sterilization may have some impact on the detectable levels of endotoxin on a product, this is not the only factor that must be considered. Critical to determining if a sample may be tested pre- sterilization is the product’s ability to support microbial growth, as growth in bacteria that are present before sterilization would likely equate to a rise in endotoxin levels.  

A validated sterilization cycle is, by definition, effective in eliminating viable organisms. Therefore, we would not expect microbial growth or an increase in the amount of endotoxin, between cycle completion and endotoxin testing. For this reason, endotoxin testing is typically performed post-sterilization. However, per ANSI/AAMI ST72:2019, pre-sterilization testing may be performed, with documented justification, if a sample does not support microbial growth.

If the sample supports microbial growth, endotoxin levels cannot be expected to remain consistent. Results obtained from an initial timepoint may be lower than those present at a later timepoint due to potential growth. This may cause results below the endotoxin specification to be obtained at one point while the finished, sterilized product may exceed the same specification and be considered unsafe for use. For this reason, pre-sterilization testing should not be considered as an option for samples where microbial growth may occur.  

If it is unknown if the product supports microbial growth, bioburden testing may be performed in accordance with ISO 11737-1. 

For more information or help with an evaluation or justification for pre-sterilization endotoxin testing, please reach out to our Expert Advisory Services (EAS) team at [email protected].  

Peter Cornelis, Ir.

Senior Expert Microbiology

Peter Cornelis graduated from the Catholic University of Leuven (Belgium) in 2000 as a Master in Applied Biological Sciences (Major Biotechnology). In 2003 he started working for Toxikon Europe (now Nelson Labs) as a study director Microbiology and in-vitro Toxicology. From 2007 until 2016, he was department supervisor for Microbiology and in-vitro Toxicology. Since 2016, he is responsible for research, validation, and development of new microbiological and in-vitro toxicological methods. Peter is a member of the ISO committee TC 194 WG5, Cytotoxicity and WG 8, Irritation and sensitization. As an expert, he was involved in the adaptation of ISO/TC 194 ISO/DTS 11796:2022(E) Biological evaluation of medical devices — Guidance for interlaboratory studies to demonstrate the applicability of validated in vitro methods to assess the skin sensitization of medical devices.

Nathan Pett

Study Director BET

Nathan Pett graduated from Utah State University in 2018 with a bachelors in Biochemistry. In 2019 he began work for Nelson Labs as a Lab Analyst and in 2021 transitioned to the role of Study Director in the BET department.

Emily Spackman, B.S., RM(NRCM)

BET Consulting Study Director

Emily Spackman has worked for Nelson Laboratories in the Bacterial Endotoxins Lab for over 16 years. She graduated from the University of Utah and holds a bachelors degree in Biology. She is a member of The National Registry of Certified Microbiologists and has experience with both medical devices and pharmaceutical products.