Common sterilization indicators for autoclaves

Autoclave sterilisation – chemical indicators

Sterilization and Autoclave – Indicators

Autoclave indicators are used to validate that the autoclave is working properly.

Chemical autoclave indicators

Chemical indicators respond to a certain chemical or physical change that occurs inside the autoclave. There are several different types of chemical indicators that are used; however, chemical tape is quite popular.  Tape indicators make use of heat sensitive ink which changes color from white to a visible pattern if the temperature has reached 121 °C. If the tape doesn’t change color, that indicates that there was a problem in the sterilization process. Note that even if the tape does change color, that doesn’t prove sterilization. Changing color simply means that the outside of the item you put tape on made it to 121 °C. It says nothing about what temperature was achieved internally.

Biological autoclave indicators

Another form of autoclave indicator makes use of biological indicators. These are standardized populations of resistant bacterial spores such as Geobacillus stearothermophilus. This bacterium comes in spore form on strips or as a suspension in prepackaged vials. This bacterial spore test is used to determine if the sterilization cycle parameters were sufficient to kill the test microorganisms. This biological indicator of sterilization success is a good one and should be done every time you autoclave.  It is best to place the biological indicator in an item to be autoclaved and best if placed at the center of the load.

When should autoclave and sterilization indicators be used?

Chemical indicator tape should be used with every sterilization process to confirm that 121 °C was achieved. Although it doesn’t prove that the item is sterile, is provides a good visible input such that you know the item has been exposed to the desired heat in an autoclave. Biological indicators should be used every time you autoclave, particularly if you are generating biological waste of some type.

How are biological indicators used?

When using prepackaged vials of Geobacillus stearothermophilus, you’ll need to retrieve the vial from the item you placed it with.  Often attaching a string to the vial will help you remove the vial with ease.

The vial will be hot and under pressure so wait for it to cool for about 10 minutes. Crush the viral in the tool that is provided by the manufacturer. Incubate the spore tube and a positive control tube (bacterial vial that hasn’t been autoclaved) in an incubator at 55 °C for the amount of time suggested by the manufacturer. When incubation is complete, check the original autoclaved vial for a color change. If the spores have been deactivated, there should be no color change.  If the original autoclaved vial changes color or the liquid becomes cloudy, this indicates that the autoclaving failed and you have bacterial growth.  If the control vial does not go through a color change, that means that the control vial was not viable to begin with.

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