A sterile product is one that is free of viable microorganisms. Items produced under controlled manufacturing conditions can, prior to sterilization, have microorganisms on them, although ordinarily in low numbers. Such products are by definition, non-sterile. The purpose of radiation processing is to destroy the microbiological contaminants on these non sterile products. The destruction of microorganisms by physical and chemical agents follows an exponential law. Accordingly, one can calculate a finite probability of a surviving microorganism regardless of the delivered sterilization dose or treatment. The probability of survival is a function of the number and types (species) of microorganisms present on the product (bioburden), the sterilization process lethality, and, in some instances, the environment in which the organisms exist during treatment. It follows the sterilized cannot be ensured in the absolute sense. A sterility assurance level (SAL) is derived mathematically and it defines the probability of a viable microorganism on an individual product unit.

The primary manufacturer has ultimate responsibility for ensuring that all sterilization, operations and quality assurance checks used for the product are appropriate, adequate and correctly preformed. However, the irradiator operator is responsible for delivering the required dose within the validated process specifications.

It is not feasible to use sterility testing of product to substantiate a sterility assurance level (SAL) of less than 10^-2 (i.e. 10^-3, 10^-4, 10^-5, 10^-6 ) because of the high number of test samples that would be required to substantiate the SAL. For example, in order to prove an SAL of 10-6, one million items would need to be sterility tested after exposure to the sterilization process. This is impractical since typically the sterility test false level that can be achieved is 1/1000 or 0.1%. Therefore, the use of product sterility testing is not recommended for process monitoring/release of product.