Bowie-Dick & Vacuum Leak Tests: Autoclave Sterilization Cycles

Key Takeaways

• A Bowie-Dick test demonstrates proper air removal from the chamber of a pre-vacuum autoclave.
• To run a Bowie-Dick test, warm up your autoclave, place a test pack in the chamber, and initiate a test cycle.
• A test pack that’s a uniform dark black color indicates a successful cycle. No color change or partial color change indicates a failed test cycle.
• A vacuum leak test determines the air-tight integrity of a pre-vacuum autoclave’s chamber and plumbing.
• A typical vacuum leak test cycle consists of three pressure pulses, followed by a 15-minute dwell time at deep vacuum.

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 printout, 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 originally consisted of a stack of folded towels with a chemical indicator sheet at the center. Today, instead of towels, most facilities use small disposable packs made of thermochromic (temperature-sensitive) paper sandwiched between porous substrates and reticulated foam.

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Before running a Bowie-Dick test, warm your autoclave with a five-minute sterilization cycle. This clears air pockets, which could skew your test results, from the jacket.

Once the autoclave is warm, place a pack inside the empty chamber 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.

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.

 

Facilities should perform a Bowie-Dick test at the beginning of every day, after a warmup cycle but before the first processed load. If the Bowie-Dick test should fail, check the autoclave and the facility’s utilities, including water and steam quality. Non-condensable gases, which are often caused by leaking pneumatic valves and high bicarbonate levels in softened water, can prevent proper air removal. If your tests fail intermittently with no obvious mechanical cause, steam and water quality testing can help you determine whether your utilities are the problem.

Do not use any pre-vacuum cycle with confidence until you’ve repeated the Bowie-Dick test with a passing result.

What to Do If Your Test Fails

A failed Bowie-Dick test indicates that steam did not fully penetrate the pack, but it does not establish why. Before escalating the issue, rule out procedural explanations, such as a pack past its expiration date or one that’s been stored improperly, a missed warm-up cycle, or a pack placed anywhere other than the lowest shelf above the drain.

If the test fails even after you’ve addressed these issues, it could point to an issue with the chamber or the plumbing’s ability to hold a vacuum, which warrants a vacuum leak test.

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.

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 1 mmHg/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 regularly 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.

What Causes Vacuum Leaks?

Most vacuum leaks are caused by worn components, such as the door gasket and its sealant; the chamber, jacket, exhaust, and water valve assemblies; the vacuum release assembly with its check valve and air filter; and/or the chamber and jacket steam traps. Consolidated’s SterilCare preventative maintenance kits ensure you have these replacement parts on hand should any of these components fail.

These components do not wear at the same rate. For example, steam valves degrade faster than valves that handle air or water. We advise that you evaluate and replace sterilizer parts annually or every 300 cycles, whichever comes first, and schedule routine preventative maintenance to prevent emergency repair calls.

Which Autoclaves Require a Bowie-Dick or Vacuum Leak Test?

Bowie-Dick and vacuum leak tests are both designed to gauge the performance of a vacuum system, which limits them to pre-vacuum autoclaves, units equipped with either a water injector or liquid ring vacuum pump to mechanically remove air from the chamber.

Gravity sterilizers do not rely on dynamic air removal, which means there’s nothing for a Bowie-Dick test to assess, and no chamber vacuum for a leak test to hold. Instead, steam entering the chamber displaces ambient air through a drain vent without mechanical assistance. Although some gravity autoclaves are equipped with a post-vacuum cycle to dry the load, this takes place after sterilization is complete and has no impact on how air is removed during the cycle itself.

If your facility uses gravity sterilizers, you should instead monitor performance using chemical indicators, biological indicators, and control system records.

What to Record When Running a Bowie-Dick or Vacuum Leak Test

Laboratories and healthcare facilities alike should maintain detailed autoclave records, with a dedicated log sheet for each sterilizer and a retention period of roughly three years, subject to your facility’s own requirements.

For a Bowie-Dick test entry, log the date, operator, and result. For a vacuum leak test entry, note the numeric leak rate displayed on the control system screen. A rate that steadily increases but does not exceed your acceptance threshold could indicate a developing problem, so it’s important to keep close tabs on and give your service team somewhere to begin should the unit eventually need attention.

Why You Can’t Always Rely on a Passing Result

An autoclave that consistently passes Bowie-Dick and vacuum leak tests can still produce improperly sterilized loads. If you’re experiencing issues with your autoclave, it could be due to poor steam quality or improper calibration.

Steam that contains non-condensable gas above 3.5% by volume, or with a dryness value below the accepted threshold, will reduce sterilization efficacy even when air removal is functioning properly, and the temperature reading on your control system might not show superheat, even if it’s present. Therefore, it’s important to test steam quality regularly to identify and address deviations from acceptable ranges that could contribute to improper sterilization.

Calibrating your autoclave is equally important. An autoclave whose temperature sensors have drifted from a National Institute of Standards and Technology (NIST)-traceable reference standard can pass a Bowie-Dick test while running cycles below the temperature its display reports. Our 12-step calibration procedure will help you ensure your autoclave’s sensor response falls within your specified range.

Ultimately, Bowie-Dick and vacuum leak tests are operational checks that you should treat as supplements to an autoclave validation program to ensure consistent sterilization.

Should you have any questions about autoclave testing, qualification, or validation, don’t hesitate to reach out to the Consolidated team.

Frequently Asked Questions

Can a Bowie-Dick test pass while a vacuum leak test fails?

Yes, you can get a passing Bowie-Dick test even if your vacuum leak test fails because both tests measure different conditions. Air removal during the pre-vacuum phase may be sufficient for full steam penetration even if the chamber and its plumbing admit air faster than your acceptance criteria allow. Treat a failed leak test as an early maintenance signal rather than evidence of a sterilization problem.

Does a failed vacuum leak test mean my loads weren’t sterile?

No, a failed vacuum leak test does not, by itself, indicate your loads were unsterile. The test measures how much vacuum the autoclave chamber and plumbing lose over a set period, which reports on mechanical integrity, not microbial kill. As long as your Bowie-Dick test and biological indicator results continue to pass, your sterilizer is achieving sterilization.

Do modern autoclaves with air detectors still need Bowie-Dick testing?

Modern pre-vacuum autoclaves — even those fitted with an air detector — still require daily Bowie-Dick testing. Non-condensable gas can collect anywhere inside the chamber, and catching it with sensors alone would require positioning them exactly wherever the gas happens to sit. A test pack placed at the coldest point of the chamber captures the entire load space instead of a few fixed measurement points.

Can you run a Bowie-Dick test with a load in the chamber?

No, a Bowie-Dick test requires an empty chamber. Additional items can alter the thermal mass and airflow resistance inside the autoclave chamber, reducing the test’s sensitivity and potentially masking air leaks.