When sterilizing solids (e.g. instruments, empty glassware, pipette tips in boxes, filters and textiles) as well as when decontaminating waste in destruction bags it must be ensured that a steam atmosphere is building exactly where it is required. Namely on and inside the product to be sterilized. Many autoclaves do not remove air reliably from the autoclave and from the product. If air remains inside the autoclave and the product, proper sterilizing is not possible, as only steam transports the thermal energy required to reliably deactivate micro-organisms.

Ineffective air removal

Illustrations 1 and 2 show ineffective air removal using the example of a box with pipette tips as well as destruction bags. If the autoclave is just simply heated up, air is displaced and a steam atmosphere builds up inside the pressure vessel of the autoclave but air remains inside the product to be sterilized. Air remaining inside the product, however, prevents the steam from entering where its thermal energy is needed to achieve a sterilizing effect.

At the same temperature as steam (e.g. 121°C), air contains only a fraction of the required thermal energy. For products that cannot be sterilized within a steam atmosphere, there are hot-air sterilizers - however, they sterilize at higher temperatures (180 - 250⁰C) requiring a lot more time (up to several hours sterilization time). A sterilizing effect of air at temperatures of 121°C - 134°C and a sterilization time of 3-20 that are usually used inside a steam sterilizer is therefore in praxis not given.

Ill.1 – Ineffecive air removal

Ill.2 – Ineffecive air removal

Ill.3 – Effecive air removal with pre-vacuum

Effective air removal

For a complete and reproducible removal of air from the autoclave and the product to be sterilized, a fractionated pre-vacuum is to be used. Therefore, the autoclave is equipped with a vacuum system. During the heat-up phase, vacuum cycles take place for effective air removal followed by steam injections. Usually, a threefold fractionated pre-vacuum is applied - depending on the product, however, even more fractions may be necessary.

Drying of solid objects

Solids such as instruments or empty glassware are usually put inside a drying oven following the sterilization process. State-of-the-art autoclaves allow for drying solids directly after the sterilization process. Sterilization and drying in one process. Further handling of the sterilization material possibly causing recontamination is not necessary.

Sterilization of biologically hazard substances

Sterilization of biologically hazardous substances is a special challenge. During the heating-up phase, the air inside the autoclave is replaced with steam. Air is displaced from the autoclave and released into the room where the autoclave is installed. TRBA 100 – Technical Regulations for Biological Working Materials requires that, in labs from Security Level S2, process exhaust air from an autoclave must be treated as the exhaust air may be contaminated by micro-organisms from the product to be sterilized. An appropriate process must be used. In the case of autoclaves, it is usually filtration. Therefore, the autoclave can be equipped with an air exhaust filter. All air displaced from the autoclave passes the filter with micro-organisms being retained in the filter. The filter is sterilized "in-line" during the sterilization process to deactivate the micro-organisms retained therein.

Autoclave exhaust filtration

TRBA 100 deals only with exhaust air released from the autoclave, but not the condensate collected. During the sterilization process, steam condenses on the product and thus turns again into water (condensate). This water may also be contaminated with micro-organisms. Therefore, the condensate must remain inside the autoclave during the sterilization process and must also be sterilized "in-line" as well before being drained upon successful sterilization.

Autoclave touch screen

Qualification and validation

During qualification it is verified, whether a device is suitable for its intended use and whether a process - e.g. a sterilization process - can be performed considering the product to be sterilized with a continuing (reproducible) result, a sterile product. Generally, the qualification process is split into three basic parts:

  1. IQ – Installation Qualification Verification, whether a device was manufactured and installed according to specifications.
  2. OQ – Operational Qualification Verification, whether a device operates generally according to specified functions.
  3. PQ – Performance Qualification Verification, whether a device with product to be treated functions according to specifications.

Target of Qualification and Validation is a documented proof that a device is suitable for its intended use.

The sterilizing effect of an autoclave process is verified during OQ (empty chamber) and PQ (with product) by means of external data loggers for temperature and pressure as well as using bio-indicators based on Bacillus Stearothermophilus. Whereas external data loggers for temperature pressure provide evidence that the control of the autoclave displays and documents reliable values as well as performs the sterilization process within defined tolerances, bio-indicators provide evidence as to biological efficiency. To place bio-indicators, it must be defined for which areas of the product to be sterilized it is most difficult to achieve biological efficiency. Accurately in those areas, bio-indicators must be placed to cover a "worst-case scenario". All steps of an IQ, OQ and PQ must be documented in detail. In each and every case of carrying out an IQ, OQ and PQ, close cooperation between user and manufacturer is required.

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