INFECTION PREVENTION & CONTROL
determine the process time that will achieve, for example, a 6-log reduction, and then simply to double the cycle time to achieve a theoretical 12-log reduction. For hospital processes – unlike industrial sterilisation processes – it is impractical to measure bioburden on the huge range of diverse reusable medical devices and use this to define the sterilisation process. In these circumstances, it is more practical and appropriate to use the overkill method to define the sterilisation process. Here, as the total bioburden upon the load is not known, a worst-case estimate is used, and the sterilisation process set according to this scenario.
This entails a number of assumptions are made. The first assumption is that the load will not contain more than 1 million microorganisms; the second assumption is that the resistance of those microorganisms to the sterilisation process is lower than the reference organism for that particular process. The output of this is a sterilisation process that will deliver a 12-log reduction (sometimes referred to as a 12-D cycle) and thus deliver the appropriate assurance of sterility - a theoretical probability of there being a viable microorganism present that is equal to or less than 1 x 10-6 (one in a million probability).
Sterilisation processes must be first validated to ensure that the requirements for the intended application have been fulfilled. Once the sterilisation process has been established, it is then necessary to monitor that process to ensure that the performance
Biological indicators (BIs) are test systems containing viable microorganisms (typically bacterial spores) that provide a specified resistance (challenge) to the relevant sterilisation process.
level required is continually achieved. The principal methods used are biological monitoring, and parametric monitoring.
Biological monitoring
Biological indicators (BIs) are test systems containing viable microorganisms (typically bacterial spores) that provide a specified resistance (challenge) to the relevant sterilisation process. As well as being used during validation, they can also be used to monitor the sterilisation process; these BIs are incubated after processing to verify that the reference microorganism has been inactivated. The advantage of using BIs is that they utilise actual resistant microorganisms and hence assess the sterilisation process from a microbiological perspective.
The disadvantages of using BIs for routine monitoring is that they typically contain only 6 logs (one million) microorganisms; as described above, many sterilisation processes are designed to deliver a lethality equivalent to a 12-log reduction – hence BIs are not monitoring the entire sterilisation process. BIs also must be incubated after
use, so an immediate result is not possible. BIs are also used in combination with chemical indicators (CIs); while not based on biological inactivation, CIs have the advantage of being typically more consistent than BIs, but may not be directly correlated to microbiological kill, but can be correlated to the physical variables of the sterilisation process, and give an instant result. With parametric monitoring physical variables that are essential to the microbiological efficacy of the sterilisation process are monitored to ensure that they are within pre-set values and their respective tolerances. The parameters associated with these variables can then be used to assure the efficacy of the process. When used in this way, the processed load can be released as sterile, based on records of these parameters, a technique known as parametric release. One advantage of this approach is that there is little delay in releasing the load; the load can be released after a review of the sterilisation process records. This approach does, however, place a lot of emphasis on periodic validation to ensure that the requirements for the sterilisation process are being fulfilled. Parametric monitoring is also used in combination with chemical indicators (CIs); CIs do not give quantitative information, but they do have the advantage that they can be placed in locations that physical sensors cannot, thus assuring consistency of attainment of the physical variables throughout the load.
Monitoring approaches
The nature of the type of monitoring used to verify attainment of a sterile product is dependent upon many factors; whether the load is being processed industrially or in healthcare may bias the preference for the monitoring approach used, however it is not uncommon for a monitoring approach to include biological, chemical and parametric approaches. ISO, the International Organization for Standardization, has a series of standards dedicated to validation and routine control of sterilisation processes. ISO 149371
gives
generic requirements for how to monitor sterilisation processes. Other ISO standards give more prescriptive requirements for specific sterilisation methods such as, for example, steam (ISO 17665),2 oxide (ISO 11135),3
ethylene radiation (ISO 11137)4
these standards are then used by Notified Bodies as part of the conformity assessment procedure.
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