7 Common Causes for Failure of a Bowie-Dick Test

Sterilization departments have one of the most important responsibilities within a healthcare facility — ensuring that all surgical instruments and other reusable equipment are sterilized and safe to use during upcoming procedures. Without these services, operations at hospitals and ambulatory surgery centers would grind to a halt.

In laboratory settings (especially in ones that carry out infectious disease research), steam sterilization is equally important — playing a key role in preventing the spread of pathogens and maintaining the integrity of samples and experiments.

So whether you work in a lab or sterile processing department, it’s crucial to make sure that your autoclaves are always working properly. One way to do this is by running a Bowie-Dick test. Read on to find out more about this test, including how to conduct one and how to identify different failure cases.

What Is a Bowie-Dick Test?

The Bowie-Dick Test is a standard operational test by which hospitals and laboratories can demonstrate proper air removal from their pre-vacuum autoclave chamber. It is primarily useful for testing pre-vacuum cycles that are sterilizing wrapped goods or packs. As we’ve mentioned in previous blog posts, pockets of cool air act as a barrier that prevents steam penetration into these packs, thus inhibiting sterilization.

The first test pack was developed in England by doctors J.H. Bowie and J. Dick in 1963, and in the time since has become the standard for confirming suitable autoclave performance. Today’s test packs include thermochromic paper to determine whether steam has suitably penetrated the porous load. A successful test shows a uniform, dark black color pattern on the paper and means that a successful vacuum and full steam penetration was achieved. No color change or a partial color change indicates a failed test.

How To Run a Bowie-Dick Test

Running a Bowie-Dick Test requires placing a small, disposable test pack into the autoclave and running a four-minute sterilization cycle, typically a Bowie-Dick Cycle. This sterilization cycle usually consists of three to four prevacuum pulses, each of which involve injecting steam into the autoclave and then pulling out the air and steam through a vacuum, before reaching the set point of 270°F (or 132°C).

Taking a closer look, the process can be further broken down into the following steps:

• Perform a warm-up cycle. This step ensures that the autoclave is sufficiently heated up, which helps prevent false-fail test results.
• Confirm that the autoclave chamber is empty. Bowie-Dick tests are always run in an otherwise empty autoclave. This makes it easier to detect air leaks or problems with its air-removal system as the presence of additional packs reduces the sensitivity of the test.
• Place the test pack on the lowest shelf above the drain. The pack must be placed in this specific location because the drain is where the vacuum is drawn — putting it in the natural path of any residual air exiting the chamber.
• Begin the Bowie-Dick test cycle. While parameters for these cycles aren’t specifically defined in the ANSI/AAMI ST79, general guidance recommends three-and-a-half to four minutes of exposure time. However, many modern autoclaves have pre-programmed cycles that are specifically designed for running Bowie-Dick tests.
• Examine the test sheet and interpret results. Once the test cycle is complete, open the pack to see the results.

Bruce Gillingham, resident autoclave expert of Consolidated Sterilizer Systems, believes it is an imperative process that should be performed each and every day within a hospital setting. This is because many hospitals (and labs) end up shutting down their autoclave at the end of the day, which can lead to unexpected issues when it is turned on again the next day.

“We recommend running a routine 5-minute sterilization cycle at the start of every day. After this is completed run the Bowie-Dick Test Cycle,” said Gillingham. “The initial 5-minute cycle helps to get rid of any air pockets that might be in the jacket. Once this is complete, we recommend running a Bowie-Dick Test before you sterilize your first load of the day.”

Interpreting the Test Results

“It’s the color change that tells you the steam is penetrating the test pack and pulling all the air out of the autoclave,” according to Gillingham.

A successful Bowie-Dick Test is very evident based on the color change of the thermochormatic paper within the test pack. If the thermochromatic paper turns completely black, then the steam has penetrated the load and the autoclave is operating correctly.

Lack of or a partial color change indicates an unsuccessful Bowie-Dick Test cycle. “If the color hasn’t completely changed, it means something hasn’t worked correctly,” said Gillingham.

7 Common Causes for Bowie-Dick Test Failure

Whether it’s because of problems with the autoclaving unit, the test pack, or a result of human error, there are several explanations for why a Bowie-Dick test might produce a failed result. These are seven of the most common:

1. Air Leak: If air is able to leak into the autoclave chamber, the steam will be unable to penetrate the load to the point of total sterilization.
2. Unwanted Condensation: Occasionally, condensation gets trapped in the jacket of the autoclave, which can lead to cold spots at the base of the unit. This could also indicate a wet-steam issue.
3. Faulty Test Pack: From time to time, a Bowie-Dick test pack can be faulty.
4. No Warm-Up Cycle: A warm-up cycle enables the sterilizer chamber and jacket to reach temperature; not running a warm-up cycle can cause a failure.
5. Incorrect Procedures: Test packs work under very specific conditions, so it’s important to make sure all steps are followed exactly when running the test.
6. Low Vacuum Level: For a sterilization cycle to run successfully, an autoclave must create sufficient vacuum conditions. If the unit is incapable of producing these conditions, sterilization will not be achieved.
7. Presence of Non-Condensable Gas: Non-condensable gasses decrease sterilization efficacy and can make their way into the chamber via pneumatic valve leaks or water that has elevated levels of bicarbonate.

The good news? There are measures you can take to prevent these causes from occurring. Take a look at the following chart for some possible solutions.

Cause of Failure	Possible Solution
Air Leak	Run a Vacuum Leak Test to further determine if an air leak exists or not.
Unwanted Condensation	Check the steam traps on the autoclave. Check steam quality and wetness.
Faulty Test Pack	Check the expiration date and make sure the packs are being stored in the proper environment.
No Warm-Up Cycle	Run a 5 minute sterilization cycle prior to running the Bowie-Dick Cycle.
Incorrect Procedures	Test packs should be placed in an empty chamber directly over the drain on the bottom rack or shelf. They are designed for use at 270°-273°F (132°-134°C).
Low Vacuum Level	Check the vacuum pump and pressure switch, as either (or both) may be faulty. Another culprit could be the vacuum pump’s water supply — water that’s too hot may cause the pump to malfunction.
Presence of Non-Condensable Gas	If a leaky pneumatic valve is the issue, it will need to be repaired or replaced. Steam quality testing may also be in order, especially if you rely on a softened water source.

 

Next Steps after a Bowie-Dick Test

A successful Bowie-Dick Test means you can begin sterilizing loads with a prevacuum cycle. Meanwhile, a failed Bowie-Dick Test means further testing is required. If none of the above solutions (see table) work then both the autoclave and the utilities should be checked. The autoclave should not be used with any confidence until the test is repeated with a passing result.

Please contact us with any questions or if you would like more information about running Bowie-Dick tests or interpreting the results. If you’re looking for test packs for your sterilizers, you can also visit our autoclave supply shop to stock up.