Sterility is critical in medical device manufacturing, but the definition of “sterile” is more technical than absolute. In highly regulated environments like medical device manufacturing, the definition of “sterile” must be both measurable and defendable. That’s where the concept of a Sterility Assurance Level (SAL) comes in. This article breaks down what SAL means, how it’s validated, and why 10⁻⁶ is the gold standard across most global markets.
What is a Sterility Assurance Level (SAL)?
A sterility assurance level, or SAL, is a rating as to the probability of a nonsterile device occurring for a given validated sterile product and manufacturing line. Typically expressed as 10-x, the most commonly used SAL in the medical device industry tends to be 10-6, which translates into a probability of having a single nonsterile device out of one million units produced.
How is an SAL established?
An SAL is established through a process called sterilization validation. A sterilization validation involves understanding the typical microbial load associated with the manufacturing process for a specific product. Once this is understood, there is a defined process for setting up a sterilization process to assure a target SAL. ISO standards exist for different sterilization modalities (e.g. ISO 11135 for ethylene oxide sterilization and ISO 11137 for radiation sterilization.)
How Does Radiation Sterilization Validation Work?
ISO 11137 outlines several methods for validating a radiation sterilization process. Some approaches aim to calculate the minimum dose required to meet sterility, optimizing for material compatibility and product performance. Others assume a commonly used dose (like 20 or 25 kGy) and focus on substantiating that this dose reliably delivers the target SAL. While substantiation by VDmax may result in a higher sterilization dose than Method 1 or Method 2, it’s often faster and more cost-effective to implement due to smaller sample sizes requiring less destructive testing.
Is a Sterilization Validation a One-Time Thing? Is There Maintenance Involved?
Since SALs are predicated on manufacturing and sterilization processes that can drift and change over time, quarterly audits are typically performed to check that the numbers and types of microorganisms have not resulted in a change in radiation resistance compared to the original validation. Over time, with demonstrated stability in manufacturing cleanliness, ISO 11137-1 allows dose audits to be performed less frequently.
Are other SALs possible or is 10-6 the standard number?
The FDA considers “alternate SALs” using a risk-based approach which might include justification that benefit of the product being available to patients outweighs risk of the device not surviving a more challenging sterilization process, or the impact of nonsterility being lower for products that only contact nonsterile parts of the body.
From Definition to Application
While the math behind SAL may seem abstract, it’s central to how manufacturers, regulators, and sterilization partners define success. Understanding how SALs are set, validated, and maintained over time is key to ensuring product safety, meeting regulatory expectations, and maintaining consistent quality in sterilization processes.
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