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INFECTION PREVENTION & CONTROL


Alcohol toBIornot toBI? That is the question…


There are a number of ways to monitor the sterilisation process to ensure that it is successful. The two principal methods used are biological and parametric monitoring. Richard Bancroft, science & technical director at STERIS Corporation explores the pros and cons of each method.


Sterilisation is a science that is not clear until the fundamental principles of the combination of physics, microbiology and probability are understood. Sterility is dependent upon a given set of sterilisation process conditions that are usually steady state (so that they can be more readily controlled); has limitations, based on the number and resistance of the microorganisms to be destroyed; and is assessed based on probability, hence cannot be easily measured in practical terms, and is often derived from theoretical considerations. We accept that the literal meaning of sterile, free from all viable microorganisms, is somewhat utopian, and the practical reality of the state of sterile is finite, hence must be expressed as a probability. Depending upon the type and nature of the sterilisation process, there are different ways of controlling and verifying the process to assure a sterile load is produced. These different methods of sterilisation process monitoring are based on science, historical practice, or a combination of both.


Industrial processes vs hospital processes


Industrial sterilisation processes are typically different to those found in hospitals or healthcare, although the end result can be considered to be identical, the way of


JUNE 2018


getting a product to be sterile may be fundamentally different.


The length or amount of lethality of a given sterilisation process can be significantly reduced, but achieve the end result, if the bioburden of the load to be processed is initially low, and most importantly, known and consistent. This bioburden approach is commonly used in industry, where loads are standardised and consistent from one day to the next.


The processing of the load and its raw materials can be microbiologically understood and controlled, with the bioburden assessed, such that a conservative estimate of the total load bioburden can be made, and the sterilisation process adapted (or, in reality, reduced) in order to be able to safely sterilise the load without unnecessary time or damage to the load. This can be especially important in drug/medical device ‘combination products’, where damage or degradation to a pharmaceutical active can occur during the sterilisation process; as these processes may


have an excessive amount of safety margin, which normally is not a consequence, in these circumstances, delivering a ‘safe’ minimum sterilisation process is highly desirable. Industrial sterilisation processes also typically use a validation technique known as a half-cycle (or fractional-cycle) approach, in order to establish the lethality of a given sterilisation cycle; in order to achieve the necessary probability of a non-sterile unit (NSU), which is almost always a probability of 1 NSU per million of units processed, a log reduction in the order of 7 to 12 logs may be necessary.


Demonstrating this log reduction using a population of microorganisms is practically very difficult, due to the huge numbers of microorganisms involved. For example, a 12-log population is a trillion (one million million) microorganisms, or one million biological indicators, each with a population of one million microorganisms – greater than a cubic metre of typically sized spore strips. For these practical reasons, it is possible to


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