Sterility assurance level

Sterility assurance level (SAL) is a term used in microbiology to describe the probability of a single unit being non-sterile after it has been subjected to the sterilization process. For example, medical device manufacturers design their sterilization processes for an extremely low SAL - "one in a million" devices should be nonsterile. SAL is also used to describe the killing efficacy of a sterilization process, where a very effective sterilization process has a very high SAL.

In microbiology it's impossible to prove that all organisms have been destroyed because: 1) they could be present but undetectable simply because they're not being incubated in their preferred environment and 2) they could be present but undetectable because their existence has never been discovered. Therefore SALs are used to describe the probability that a given sterilization process has destroyed all of the microorganisms.

Terminology
Mathematically, SALs referring to probability are usually very small numbers and so are properly expressed as negative exponents ("The SAL of this process is 10 to the minus six"). 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 10 to the six"). In this usage, the negative effect of the process is sometimes inferred by using the word "reduction" ("This process gives a six-log reduction"). Because of this ambiguity, group discussions of SAL must define the 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. So a process shown to achieve a "6-log reduction" (10-6) will reduce a population from a million organisms (106) to very close to zero, theoretically. It is common to employ overkill cycles to provide greatest assurance of sterility for critical products such as implantable devices.

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