Autoclaving is the most effective and reliable means of sterilising laboratory materials.
For most purposes, the following cycles will ensure sterilisation of correctly loaded autoclaves:
- four minutes holding time at 134°C
- 10 minutes holding time at 126°C
- 15 minutes holding time at 121°C
- 25 minutes holding time at 115°C.
Autoclaves used for decontaminating infectious biological waste and genetically modified organisms (GMOs) need to be regularly tested to ensure the effectiveness of the autoclave.
Any autoclave that is not proven to decontaminate waste should not be used for infectious and GMO biological waste.
All pressurised vessel have to be certified by Worksafe. These records are kept by the School with the pressurised vessel.
This is not a comprehensive list of all requirements for the care and use of autoclaves, reference should made to AS/NZS 2243.3 & AS/NZS 1200.
Recommendations for the care and use of autoclaves
- The steam should be saturated and free from corrosion inhibitors or other chemicals, which could contaminate the items being sterilised.
- All materials to be autoclaved should be in containers that allow ready removal of air and permit good heat penetration; the chamber should not be tightly packed or steam will not reach the load evenly. Bags should allow the steam to reach their contents.
- For autoclaves without an interlocking safety device that prevents the door being opened when the chamber is pressurised, the main steam valve should be closed and the temperature allowed to fall below 80ºC before the door is opened.
- Operators should wear heat-insulating gloves and visors for protection when opening the autoclave, even when the temperature has fallen below 80ºC.
- Care should be taken to ensure that the relief valves of pressure cooker autoclaves do not become blocked by pater and the like in the load.
- Responsibility for operation and routine care should be assigned to trained individuals and a preventive maintenance programme should include regular inspection of the chamber, door seals and all gauges and controls by qualified personnel.
- The drain screen filter of the chamber (if available) should be removed and cleaned daily.
- In any routine monitoring of autoclave performance, biological indicators or thermocouples should be placed in several positions in a load, including those least likely to attain sterilisation conditions. This is to determine proper operating cycles. Refer to AS/NZS 2243.3 section 6.6.6. Records of these tests must be kept.
- A logbook recording details of steriliser load and cycle should be maintained.
- Standard operating procedures (SOPs) should be available for training personnel in the use and maintenance of autoclaves.
Testing the efficacy of autoclaves
Quality control is essential to ensure that potentially infectious agents are destroyed by adequate sterilisation regimes. There are a number of ways in which the efficacy of an autoclave may be tested.
Browne's tubes are glass tubes that contain heat-sensitive dyes. These change colour after sufficient time at the desired temperature.
Indicator tape is applied to articles being autoclaved. If the process has been satisfactory, dark brown stripes will appear across the tape. Pale brown stripes are suggestive of poor heat penetration, and an unsatisfactory sterilisation process. These methods give an immediate indication of the success or otherwise of an autoclave run but they are only suggestive of a successful sterilisation.
Spore strips may be placed inside the autoclave at the start of its cycle. After running, the autoclave the strip is recovered and cultured. Absence of growth after a suitable period indicates a successful run.
The problem with this method is that it is retrospective. If a problem has arisen, then this will be discovered only when the spores have germinated. This is probably too late to take effective action other than to call in an engineer to prevent further problems.