Understanding Tissue Sterilization, Part 1: Aseptic Processing vs. Terminal Sterilization

When you need to procure allograft tissue, you want to be confident that it has gone through appropriate procedures to minimize the risk of infection. Any microorganisms present on a graft can cause infection in the patient, so reducing the chance of infection is crucial. You will see products labeled with phrases such as “aseptically processed” and “terminal sterilization.”

But what do these terms mean, and how are they related to contamination risk?

Aseptic processing minimizes the chance that donor tissues can be contaminated during preparation. Aseptic processing involves specific, validated protocols which include mechanically agitating the tissues to remove blood, lipids, and cells; cleaning them with detergents, antibiotics, antifungals, or chemical disinfectants; and cutting/shaping them in a clean room. These protocols are validated through biological testing of the tissues and environmental testing of the equipment and packaging used. While aseptically processed allografts are prepared and tested in ways which reduce the microorganisms that could be present, aseptic processing is not the same as terminal sterilization.

Terminal sterilization is an additional, final procedure completed by some tissue banks on some allografts. It’s good to know that terminally sterile allografts have also undergone aseptic processing. After aseptic processing, the grafts are placed in their final packaging before sterilization. Most often, sterilization is achieved through exposure to a validated low dose of gamma or electron beam radiation, which is administered by an ISO-certified third party.

Terminal sterilization destroys virtually all microorganisms that may be present, including sporicidal organisms and viruses which cannot be adequately destroyed by aseptic processing. Terminal sterilization is regarded as the highest safety standard for allograft tissue. A major difference between aseptically processed and terminally sterilized tissue is the risk of contamination. What are the chances of a microorganism surviving aseptic processing compared to terminal sterilization?

The Sterility Assurance Level (SAL) is the probability of one microorganism surviving on the tissue. According to the Centers for Disease Control and Prevention, terminally sterilized tissue has a SAL of 10-6. This means that there is a one in 1,000,000 chance that one microorganism will survive the sterilization methods and be present in the sterilized tissue. A SAL of 10-6 assures us that the chance of infection is literally “one-in-a-million.”

Aseptically processed and cleansed tissue has a SAL of 10-3, meaning that there is a one in 1,000 chance of a microorganism surviving on the tissue. While this risk is still quite low, it is technically a much higher risk than what is faced when using terminally sterilized tissue. The CDC has stated that potentially lethal sporicidal organisms are unaffected by aseptic cleansing alone. Why, then, aren’t all allografts sterilized?

Allografts used in procedures requiring living cells cannot be terminally sterilized without negatively impacting the tissue. In these cases, the surgeon, hospital, and patient must balance the risk of infection against the need for maximum preservation of the allograft’s cellular integrity and strength. This decision is usually influenced by the medical condition being treated and the needs of the patient.

Choosing the right allograft for a procedure without understanding the difference between aseptic processing and sterilization can be confusing. To add to this confusion, a certain sterility claim on many aseptically processed tissues states “sterile based on US Pharmacopeia Guideline 71 (USP <71>)”. USP <71> mandates that “for every 100 containers, ten percent or four containers, whichever is greater” must be tested for contamination. Products that undergo this testing successfully may be said to be “sterile based on USP <71>”, but they have only been aseptically processed, not terminally sterilized.

We’ll compare the methods of sterilization used by today’s tissue banks in Part 2. For all practical purposes, when you source tissue from an accredited tissue bank that adheres to the stringent guidelines of the tissue bank accrediting body1, the risk of infection is very low, whether the tissue is aseptically processed or terminally sterilized. However, it certainly helps to understand the difference when you are trying to make discriminating decisions on behalf of your patients!

Thanks for reading!

Click on this link to get to Understanding Tissue Sterilization, Part 2: Radiation Roundup.


Guideline for Disinfection and Sterilization in Healthcare Facilities: Sterilization (2008). Centers for Disease Control and Prevention. Retrieved from

US Pharmacopeia Sterility Testing Guidelines <71>.