The Importance of Leak Tests in Steam Sterilization

The Complementary Roles of Steam Sterilizer Diagnostic Tests

Medical steam sterilizers, or autoclaves, remove air from a chamber and replace it with moist heat steam for a certain amount of time, inactivating and destroying microorganisms at a cellular level¹.

The air that needs to be removed from the chamber is a non-condensable gas; as the name suggests, this gas does not condense or break down. Any air in the autoclave chamber effectively acts like a shield between the sterilant (steam) and the sterilized surface of the device. This shield can block bacteria and other microorganisms from being exposed to the sterilant, preventing proper sterilization of the devices.

An air leak in a sterilizer's plumbing or chamber and any air trapped in the autoclave can play a significant role in sterilization failure, so testing must be conducted to ensure the equipment is functioning correctly. These diagnostic tests are:

Leak Tests to help ensure the integrity of the plumbing and vacuum system.
Bowie-Dick (Air-Removal) Tests verify that most, if not all, of the air has been removed from the chamber.

If I use a Bowie-Dick Test Pack, do I still need to conduct a Leak Test?

Yes, if you conduct a vacuum leak test, you still need to run a Bowie-Dick (Air-Removal) test, and vice-versa. These diagnostic tests are intended to complement one another and play an important role, along with biological and chemical indicators, in ensuring an effective steam sterilization process. All these tools are important in your overall sterility assurance and quality testing program.

To learn more, read our Bowie-Dick Tests and Troubleshooting Guide, which includes steps for running the test and interpreting the results.

What is a Steam Sterilizer (Autoclave) Leak Test?

A vacuum leak test is used to verify the integrity of the sterilizer pressure vessel and its plumbing. Steam sterilizer operator manuals typically recommend that the leak test be performed daily or weekly. This test intends to allow the user to proactively check the unit's integrity between service visits to see if any components may be failing.

This is best determined by keeping a log of the test results to confirm whether there has been a gradual or a more radical change in the leak rate. This will allow the user to contact their service provider to schedule a repair before a leak causes a problem with other quality tests, such as Bowie-Dick Tests, resulting in significant equipment downtime.

Documenting leak test results can be done within your facility's tracking software, such as SPM, or recording through paper methods.

A perfectly sealed vessel with perfectly sealed plumbing components will have no leaks. However, to permit reasonable wear and tear on equipment, industry standards, such as ANSI/AAMI ST8 and EN285 for medical steam sterilizers, allow up to a 1.0 mm Hg per minute leak rate average over a specified time, typically ten minutes².

What if a Sterilizer Leak Test Fails (Exceeds 1.0)?

If a leak test shows a leak rate greater than 1.0 mm Hg per minute, immediately shut down the sterilizer until the unit can be serviced. Since other diagnostic tests, such as Bowie-Dick, have limited sensitivity by comparison, the leak test can act as an early indicator of problems such as inadequate air removal.

It is important to consider the history of leak test results for the sterilizer. If, in the past, the sterilizer had very low leak rates, such as 0.2 or 0.3 mm Hg per minute consistently, and the leak rate suddenly increases to 1.3 mm Hg per minute, this indicates that a component is probably failing rapidly, and service should be called immediately.

What if a leak test value is increasing but not failing?

If the leak test result has been increasing slowly, such as going from 0.7 to 0.8 mm Hg per minute readings, then the increase in leak rate is gradual, and a service visit can be scheduled on a non-emergency basis. The operator should continue to monitor the leak rate by running the leak test in accordance with the Operator Manual and their facility policy for frequency and take immediate action if it rises above the acceptable limit of 1.0 mm Hg per minute.

Sterilizer Leak Tests vs. Bowie-Dick (Air-Removal) Testing

Running a Bowie-Dick test or a leak test does not replace one another. Both diagnostic tests are important to understanding the performance of your steam sterilizer. Similar to the importance of running sterility assurance testing with biological or chemical indicators, each test has its unique purpose.

Types of Steam Sterilizer Tests

Test	Leak Test Cycle	Bowie-Dick (Air Removal) Test	DART (Daily Air-Removal Test)	Biological Indicators	Chemical Indicators
Use	The pre-programmed sterilizer cycle verifies the integrity of the sterilizer pressure vessel and its plumbing outside the chamber	Indicates if the vacuum system is operating properly for prevacuum cycles	Pre-assembled compact Bowie-Dick test pack product	Tests if conditions were met to kill a number of highly resistant bacterial spores contained in the BI	External: Confirms exposure to sterilant Internal: Confirms that sterilant achieved penetration inside the pack
Results	Provides a numerical value – that can be tracked over time	The result (color change pattern) is interpreted to confirm the effective removal of air and steam presence inside the chamber	Result in a color change interpreted to confirm the effective removal of air and steam presence inside the chamber	After incubation, it provides a pass/fail result	Placed outside or inside packs to be sterilized and interpreted at the point of use

While an air removal test verifies whether the vacuum system is adequately removing air, the leak test determines whether air is entering the sterilizer chamber through leaks in the piping. Both these tests, along with biological and chemical indicators, are necessary to provide the appropriate confidence level in the sterility of your processed devices and should not be considered substitutes for one another.

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