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Sterility Assurance Level (SAL)

1970

SAL is a quantitative measure representing the probability of a single viable microorganism surviving on an item after sterilization. A commonly required SAL for medical devices is \(10^{-6}\), meaning a one-in-a-million chance of a non-sterile unit. This probabilistic approach acknowledges that absolute sterility cannot be proven, only inferred statistically from process validation.

The concept of the Sterility Assurance Level (SAL) is rooted in the understanding that microbial death follows first-order kinetics, meaning it is a probabilistic event. It is impossible to prove that every single microorganism on an object has been killed; one can only calculate the probability of a microbe surviving the process. The SAL is this probability. For instance, an SAL of \(10^{-6}\) does not mean one contaminated item exists for every million produced. Instead, it means there is a one-in-a-million chance for any given item to harbor a single viable microorganism after the sterilization process. This statistical approach is fundamental to the validation of sterilization cycles in regulated industries like pharmaceuticals and medical devices. To establish a cycle that achieves a desired SAL, manufacturers perform validation studies. This often involves placing biological indicators (BIs), which are standardized preparations of highly resistant bacterial spores (like Geobacillus stearothermophilus for steam or Bacillus atrophaeus for EtO), in the most challenging locations within a sterilization load. The process parameters (e.g., time, temperature, concentration) are then adjusted and tested until they consistently achieve a complete kill of the BIs. The cycle is then extended for a ‘half-cycle’ or more to provide a safety margin, ensuring the process can deliver the target SAL of \(10^{-6}\) or better. The choice of SAL depends on the intended use of the product. Items that will come into contact with compromised tissue, such as surgical implants and injectable drugs, require the most stringent SAL of \(10^{-6}\). For items that contact intact skin, a lower SAL of \(10^{-3}\) may be acceptable.

The formalization of SAL as a regulatory standard emerged in the latter half of the 20th century, driven by agencies like the U.S. Food and Drug Administration (FDA) and international standards organizations (ISO). Before this, sterility was often treated as an absolute state—an item was either sterile or not. However, as manufacturing scaled up and the risks of contamination became better understood, a more rigorous, quantitative framework was needed. This shift from a qualitative to a probabilistic definition of sterility was a major advancement. It provided a scientific and defensible basis for process validation, allowing manufacturers to demonstrate the effectiveness and reproducibility of their sterilization methods to regulators and ensuring a consistent level of safety for patients.