A sterile product item is one which is free of viable micro-organisms and a non-sterile item is one that has one or more viable micro-organisms. Good Manufacturing Practice (GMP) requires that adventitious microbiological contamination of medical product from all sources is minimized by all practical means. Even so, product items produced under standard manufacturing conditions in accordance with GMP may well, prior to radiation sterilization, have viable micro-organisms on them, albeit in low numbers. Such product items are non-sterile. The purpose of radiation sterilization processing is to inactivate the microbiological contaminants and thereby transform the non-sterile product items to sterile ones.

The inactivation of micro-organisms by radiation, in common with other physical and chemical agents used to sterilize medical products, follows exponential law; inevitably this means that there is always a finite probability that a micro-organism may survive regardless of the extent of the dose delivered. For a given dose, the probability of survival is determined by the number and type of micro-organisms being irradiated and the environment in which the organism existed during irradiation. It follows that the sterility of any one item in a population of items subjected to radiation sterilization cannot be guaranteed and the sterility of the irradiated population of items has to be defined in terms of the probability of the existence of a non-sterile item in that population. The value taken by this probability is what is generally meant by degree of sterility assurance or Sterility Assurance Level (SAL) achieved by sterilization processing.

In some countries, a maximum SAL of 10^-6 is applied in the sterilization of medical products, whereas in others difference values of SAL are being used. In selecting a maximum value of SAL, matters that have to be considered are the end use of the medical product and the extent of the risk of infection attributable to the sterilized product in this use. Usually, these matters are the purview of a national licensing or approving authority.