Definition of “Sterile”
A common definition of “sterile” is the complete destruction or inactivation of all microorganisms. This is an “absolute” definition meaning that this only happens in an absolutely ideal situation. Because an ideal situation is rarely achievable we have other standards for determining sterility in real life situations. We use a calculation called the “D-Value” and the term “Sterility Assurance Level” or SAL
- D-value
During a sterilization cycle not all microorganisms die immediately. This applies to any of our sterilizing processes. Microorganisms die gradually and at a predictable rate. This predictable rate is called the D-Value. A common definition of D-Value is the time it takes to kill 90% of microorganisms remaining on an item. Different microorganisms have different D-Values. Some die very quickly and some are much harder to kill. To test our sterilization processes we use the toughest, hardest to kill microorganisms – the ones with the highest D-Values. Geobacillus Stearothermophilus has a D-Value of about one minute when exposed to steam at 132°C. This means that if we start with, for example, 1,000,000 of them in a steam cycle 90% will die in one minute. In the next minute 90% of those that remain will die, and so on until we can say that less than one microorganism remains. Our goal in sterilization is to have less than a 1 in 1 million chance of any microorganisms remaining on our sterilized items, and our exposure times are based on achieving that goal.
- SAL
The goal of having less than a 1 in 1 million chance of microbial survival in our sterilization processes is represented by the number 10-6 and is called the Sterility Assurance Level. All of our sterilization processes, testing procedures and department policies are aimed at achieving this SAL. The SAL is a statement to our customers that our processes will kill enough microorganisms to make items safe to use in a sterile procedure. Our processes may not kill every single microorganism but so few will remain that they are not harmful to our patient or to us
Activity –
For an example of D-Value and SAL find a new case of biological indicators in your department. Inside you will find a sheet of manufacturer’s information on the function of these indicators. This sheet will state the D-Value of the test microorganisms.
Visual or Graphics
Graph of Microorganism death over a 4 minute steam cycle – or a drawing of 90% of a group of microorganisms dying in one minute, then 90% of those dying in the next minute etc.
Device Compatibility – captured in the following Factors Affecting Sterilization?
Loading Principles – captured in the following Factors Affecting Sterilization?
There are many factors affecting our ability to reach the SAL
Factor – the Microorganisms
Successful sterilization depends on being able to kill microorganisms. We can never know for sure which type might be on an item so we set all our processes to kill the toughest ones. If we can kill them then we can assume that other, weaker, ones will die too. We must also be aware of bioburden. This is the number of microorganisms present on an item. Very clean items have a low bioburden and soiled items have a high bioburden. Our cleaning processes are designed to reduce the bioburden on all items so that our sterilization processes will work well.
Factor – the Item to be Sterilized
All medical devices must be tested by their manufacturers to determine how they can be cleaned and how they can be sterilized. For example not all devices can be washed in a washer disinfector and not all can be sterilized in steam. Device manufacturers must rigorously test their devices to determine if they are compatible with common cleaning and sterilization methods. Manufacturers must validate which processes will work and provide this information with the device. We use these Manufacturer’s Instructions for Use (IFUs) to create our department work instructions.
The item must also be clean. We have stated that bioburden affects sterilization. We must reduce the bioburden on all items prior to sterilization. Most of the soil we see in reprocessing departments is protein – blood, mucus etc. with a large dose of microorganisms mixed in. Protein will coagulate when exposed to the heat of a sterilization process much as the white of an egg will coagulate when cooked. Coagulated protein will act as a shield and protect microorganisms during sterilization and many will survive the process. Soil on a processed item can never be considered to be sterile.
The way a device is packaged for sterilization will also affect the process. The material must allow the sterilant – steam or gas – to enter the package, contact the item and then allow the sterilant to be removed at the end of the cycle. The packaging material must also be compatible with the sterilizing method and act as a barrier to contamination when the item is stored or transported.
All surfaces of the item must come in contact with the sterilant. The item must be in an open position or disassembled as much as possible to allow full contact. Always follow your work instructions for disassembly.
How the devices are loaded into the sterilizer will affect the process. Here are some general guidelines for loading a sterilizer:
- Load packages so that the sterilant can be circulated around and into all packages. Allow at least 5 to 10cm between large packages and 2 to 5cm between small packages
- Do not stack items directly on top of each other
- In steam sterilization items should be placed on edge whenever possible to allow air to be removed and to prevent water from collecting. The exceptions to this are rigid containers, which are made to be placed flat; and items packaged on perforated trays which allow drainage.
- In steam sterilization peel pack pouches should be placed on edge with paper sides facing plastic sides. In gas sterilization peel pack pouches should be placed plastic side down with no over lapping.
- Packages should never touch the walls of the sterilizer.
- For steam sterilization metal rigid containers or other heavy packages should be placed on the bottom of a load with linen or lighter items on top shelves. Heavy containers can produce a lot of condensation during the cycle and small items placed below them could become very wet.
Graphics
A picture of a well loaded cart or sterilizer. Note – if we have pop-up options we could use them to identify examples of the principles above. Example – participants could click on the image where they see poor loading practices and a pop-up arrow could explain why it is not good practice.
Factor – the Sterilizer
All sterilizing methods require us to control the sterilizing agent, the exposure time, and the temperature during the cycle. These are often referred to as parameters for sterilization. We will discuss specific parameters for sterilization in other units in this series.
Most modern sterilizers have a selection of cycles with re-set temperature and times. We need to make sure that we choose the correct cycle for the devices we want to sterilize. Work instructions help us with this choice.
Complex devices may need to be sterilized using extended cycles, which have different temperatures and/or time settings. We need to follow manufacturer instructions (IFUs) for setting these extended cycles.
Our sterilizers must be maintained in good condition, and be regularly tested to show that they are capable of killing the toughest microorganisms. We use chemical and biological tests as well as specialized tests such as the Bowie Dick test in steam to monitor sterilizer function. We also monitor the printouts of each load to ensure that the cycle was correct and successful.
Regular maintenance and cleaning helps keep sterilizers running well.
Factor – the Human Element
In 1956 the American Sterilizer Company, (AMSCO), published the first volume of
Principles and Methods of Sterilization in Health Sciences
Included in their chapter on the most important factors affecting successful sterilization was the following:
The Human Element
From raw materials to purchased articles to packaged supplies ready for sterilization there is one all-encompassing factor to control – people. The performance of people is based on their knowledge and ability to follow instructions. If their actions are inadequate, incorrect or the result of emotional conflicts, established sterilization standards will be invalidated. Those who have the responsibility for producing sterile supplies should be constantly alert to behavioural characteristics associated with human error in the sterilization process.
Examples are as follows:
- Antagonistic cooperation instead of voluntary cooperation. Non-communicative with other workers who share responsibility for producing sterile supplies.
- Disregard for precautions and for adherence to sound principles in preparation and packaging of supplies.
- Refusal to follow the principles of sterilization; no interest in learning anything new or in teaching others the right way or in developing a sterile technique conscience.
- Shortcuts of sterilization cycles by manual control so as to avoid working overtime.
- Resistance to change – did it that way for twenty years without trouble.
- No judgment and tact in correction of errors, thereby inviting resentment on the part of others.
It is important to remember that sterilization is an event. It requires the maximum control of all variables so as to affect a minimum margin of doubt in the end result.
We can see from the above that personal responsibility on the part of reprocessing technicians, supervisors, and managers has always been a vital part of successful sterilization.