Steam sterilization (also called autoclaving) is conducted in an autoclave which is a container that can withstand high pressure and temperature. Steam sterilization (also called autoclaving) is conducted in an autoclave which is a container that can withstand high pressure and temperature. The basic principle of steam sterilization, as accomplished in an autoclave, is to expose each item to direct steam contact at the required temperature and pressure for the specified time.
Parameters of Steam Sterilization
There are four parameters of steam sterilization:
Steam, pressure, temperature, and time. The ideal steam for sterilization is dry saturated steam and entrained water (dryness fraction ≥97%).Pressure serves as a means to obtain the high temperatures necessary to quickly kill microorganisms. Specific temperatures must be obtained to ensure the microbicidal activity.
The two common steam-sterilizing temperatures are 121°C (250°F) and 132°C (270°F). These temperatures (and other high temperatures) must be maintained for a minimal time to kill microorganisms. Recognized minimum exposure periods for sterilization of wrapped healthcare supplies are 30 minutes at 121°C (250°F) in a gravity displacement sterilizer or 4 minutes at 132°C (270°F) in a pre-vacuum sterilizer (Table 7). At constant temperatures, sterilization times vary depending on the type of item (e.g., metal versus rubber, plastic, items with lumens), whether the item is wrapped or unwrapped, and the sterilizer type.
Bowie-Dick-type test:
Bowie-Dick (Air-Removal) tests evaluate the performance of pre-vacuum sterilizers by confirming adequate air removal from the sterilizer chamber. These air removal tests have been improved over the years, but you may be wondering, are Bowie-Dick tests still relevant in sterilizers that have programmed leak tests?
To fully answer why this test is still important today, we must first understand why it is important to remove air from the sterilizer. Air within a steam sterilizer is often referred to as a non-condensable gas (NCG). As the name indicates, non-condensable gases do not condense when touching a colder item. These gases act like a shield between the steam and the item, potentially shielding bacteria and preventing proper sterilization.
For gravity displacement sterilizers the penetration time into porous items is prolonged because of incomplete air elimination. This point is illustrated with the decontamination of 10 lbs of microbiological waste, which requires at least 45 minutes at 121°C because the entrapped air remaining in a load of waste greatly retards steam permeation and heating efficiency.
Biological Indicators:
The oldest and most recognized agent for inactivation of microorganisms is heat. D-values (time to reduce the surviving population by 90% or 1 log10) allow a direct comparison of the heat resistance of microorganisms. Because a D-value can be determined at various temperatures, a subscript is used to designate the exposure temperature (i.e., D121C). D121C-values for Geobacillus stearothermophilus used to monitor the steam sterilization process range from 1 to 2 minutes. Heat-resistant non-spore-forming bacteria, yeasts, and fungi have such low D121C values that they cannot be experimentally measured.
Steam Sterilization Validation:
This Steam Sterilization validation will review the general requirements for performing a steam sterilizer validation via the “overkill” half-cycle method as described in ISO 17665 by using chemical and biological indicators including loading patterns.
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