Autoclave Steam Sterilization Cycles, Part 9: Bowie-Dick & Vacuum Leak Test Cycle

Bowie-Dick & Vacuum Leak Tests: Autoclave Sterilization Cycles

Arthur Trapotsis
Written by: Arthur Trapotsis

The entire purpose of a laboratory or medical autoclave is to, quite simply, sterilize; however, without the proper conditions for sterility, an autoclave may fall short of its mission. Validation is an essential quality assurance measure designed to ensure that your autoclave operates exactly as intended.

There are a number of different ways to validate your autoclave, including physical monitoring using the autoclave’s display screen or print out, chemical monitoring with chemical indicators, efficacy testing with biological indicators, and operational testing.

In this article — the eighth installment of our steam sterilization cycle series — we discuss two autoclave test cycles that medical and/or laboratory personnel can use to ensure their autoclave is performing properly: the Bowie-Dick test (also known as the dynamic air removal test) and the vacuum leak test. These validation tests are used for a variety of applications, including the sterilization of medical instruments, pharmaceutical goods, and waste from biocontainment laboratories.

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What Is a Bowie-Dick Test Cycle?

While it’s not a substitute for sterility assurance testing, the Bowie-Dick test cycle demonstrates proper air removal from the chamber of a pre-vacuum autoclave. Pockets of cool air can form inside the chamber of a pre-vacuum autoclave, acting as a barrier that prevents steam from penetrating and sterilizing the load. Therefore, the air must be removed by a vacuum. This cycle is run concurrently with Bowie-Dick test packs.

A Bowie-Dick test pack consists of 29–36 huckaback towels, each folded and stacked to a height of 10–11 inches, with autoclave tape in the middle of the pack. Today, instead of towels, many facilities use small disposable packs made of thermochromic (temperature-sensitive) paper sandwiched between porous substrates and reticulated foam. These can be easily purchased online.

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To run a Bowie-Dick test, simply place a pack inside the empty chamber of an autoclave on the lowest shelf above the drain — as this is the coldest point in the chamber — and initiate a Bowie-Dick test cycle. This particular sterilization cycle consists of three to four pre-vacuum pulses before reaching the set point of 270 °F.

Line graph displaying a typical vacuum cycle with pre-vacuum pulses in a steam sterilizer.

The thermochromic paper inside the pack will indicate whether steam has penetrated the porous load. A Bowie-Dick test pack that shows a uniform dark black color pattern indicates a successful vacuum and full steam penetration, whereas no color change or partial color change indicates an unsuccessful test cycle.

Examples of an unprocessed Bowie-Dick test pack, a successful Bowie-Dick test pack, and a failed Bowie-Dick test pack.

 

You can perform a Bowie-Dick test daily, or less frequently, depending on your facility’s needs and standard operating procedures (SOPs). If the Bowie-Dick test should fail, you should check both the autoclave and the facility’s utilities. Do not use any pre-vacuum cycle with confidence until you’ve repeated the Bowie-Dick text with a passing result.

What Is a Vacuum Leak Test?

A vacuum leak test is used to determine the air-tight integrity of a pre-vacuum autoclave’s chamber and plumbing system. This test exposes the autoclave’s plumbing and components to vacuum conditions and measures how much vacuum depth is lost over a given period of time.

A typical vacuum leak test cycle consists of three vacuum/pressure pulses, followed by a 15-minute dwell time at deep vacuum. Once the vacuum leak test cycle is complete, your autoclave’s control screen will display a leak rate in units such as psia/min, kPa/min, mbar/min, or mmHG/min. Although the user determines the pass/fail criteria for a vacuum leak test based on their specifications, industry standards call for an average leak rate of 1mmHG/min or less.

How often you run a vacuum leak test cycle depends entirely on your facility’s SOPs and risk tolerance. Performing a vacuum leak test on a regular basis allows for greater confidence in the integrity of your autoclave and plumbing.

Although laboratory and medical autoclaves are calibrated upon installation, it’s important to periodically validate your pre-vacuum autoclave to verify that it is operating properly. The Bowie-Dick test and vacuum leak test are simple operational checks to incorporate into your facility’s SOPs.

Bowie-Dick Test & Vacuum Leak Test FAQs

What is a Bowie-Dick test?
A Bowie-Dick test is a standard operational test that laboratories can use to determine proper air removal from their pre-vacuum autoclave chamber and to ensure their autoclave meets the appropriate conditions for sterilization.

What is a Bowie-Dick test pack?
A Bowie-Dick test pack is a small, disposable pack made of thermochromic paper sandwiched between porous substrates and reticulated foam.

How can you tell whether a Bowie-Dick test cycle was successful?
You can tell that a Bowie-Dick test cycle was successful if the thermochromic paper in the test pack turns completely black; this indicates that steam has completely penetrated the load and that the autoclaving is operating correctly. If the pack partially changes color or does not change color at all, the test was unsuccessful.

Where should you place a Bowie-Dick test pack in an autoclave?
When running a Bowie-Dick test cycle, place the pack on the lowest shelf above the drain inside the empty autoclave chamber.

How often should you run a Bowie-Dick test cycle?
How often you run a Bowie-Dick test cycle depends entirely upon the needs of your facility, as some settings — such as hospitals or other healthcare facilities — require more frequent testing than others. Please refer to the reference document on Hospital Steam Sterilizers: AAMI ST8.

What is a vacuum leak test?
A vacuum leak test is used to determine the air-tight integrity of a pre-vacuum autoclave’s chamber and plumbing system. This test exposes the autoclave’s plumbing and components to vacuum conditions and measures how much vacuum depth is lost over a given period of time.

How do you run a vacuum leak test cycle?
A typical vacuum leak test cycle consists of three vacuum/pressure pulses, followed by a 15-minute dwell time at deep vacuum. Once the vacuum leak test cycle is complete, your autoclave’s control screen will display a leak rate in units such as psia/min, kPa/min, mbar/min, or mmHG/min.

How can you tell whether a vacuum leak test cycle was successful?
The success criteria for a vacuum leak test is based entirely on the needs of your particular facility; with that said, the industry standard calls for an average leak rate of 1mmHG/min or less.

How often should you run a vacuum leak test cycle?
Similar to a Bowie-Dick test cycle, how often you run a vacuum leak test cycle depends entirely upon the needs of your facility and your risk tolerance.

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