Sterility assurance level

Sterility assurance level (SAL) is the probability of a single unit being non-sterile after it has been subjected to sterilization.

In microbiology it is impossible to prove that all organisms have been destroyed as the likelihood of survival of an individual microorganism is never zero, so SAL is used to express the probability of the survival. For example, medical device manufacturers design their sterilization processes for an extremely low SAL, such as 10^-6, which is a 1 in 1,000,000 chance of a non-sterile unit. SAL also describes the killing efficacy of a sterilization process. A very effective sterilization process has a very low SAL.

Terminology

Mathematically, SALs referring to probability are usually very small numbers, and thus are properly expressed as negative exponents. For example, "The SAL of this process is 10^-6". SALs referring to the sterilization efficacy are usually much larger numbers and so are properly expressed as positive exponents ("The SAL of this process is 10⁶"). In this usage, the negative effect of the process is sometimes inferred by using the word "reduction" (e.g., "This process gives a six-log reduction").

Because of this ambiguity, it is important to define SAL terminology before setting standards.

SALs can be used to describe the microbial population that was destroyed by the sterilization process. Each log reduction (10⁻¹) represents a 90% reduction in microbial population. A process that achieves a "6-log reduction" (10⁻⁶) will theoretically reduce an initial population of one million organisms to very close to zero. It is common to employ overkill cycles to provide the greatest assurance of sterility for critical products such as implantable devices or injectable drugs.

SALs describing the "Probability of a Non-Sterile Unit" are expressed more specifically in some literature.