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

Autoclave Steam Sterilization Cycles, Part 8: Bowie-Dick & Vacuum Leak Test Cycle

It is important to ensure that materials processed in a laboratory autoclave have been exposed to adequate conditions for sterilization. Sterilization validation can be accomplished in a number of ways:

  • Physical monitoring through the autoclave’s display screen or print-out,
  • Chemical monitoring with chemical indicators,
  • Efficacy testing with biological indicators, and
  • Operational testing of the autoclave.

In the eighth installment of our Steam Sterilization Cycles Series, we discuss two test cycles used to ensure that a laboratory autoclave is performing properly: The Bowie-Dick Test (or dynamic air removal test), and the Vacuum Leak Test. These validation tests are used in a variety of applications, including the sterilization of medical devices, pharmaceutical goods, and waste from bio-containment laboratories.

Bowie-Dick Test Cycle

While it’s not a substitute for sterility assurance testing, the Bowie-Dick Test demonstrates proper air removal from the chamber of a pre-vacuum autoclave. As explained in earlier blog posts, pockets of cool air act as a barrier that prevents steam from penetrating the load. Therefore, the air must be removed by a vacuum.

The first Bowie-Dick test packs consisted 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, laboratories use small disposable packs made of thermochromatic (temperature sensitive) paper sandwiched between porous substrates and reticulated foam. To run the test, a pack is placed in an empty chamber on the lowest shelf above the drain (the coldest point in the chamber) and a “Bowie-Dick Cycle” is initiated. The sterilization cycle consists of three to four prevacuum pulses before reaching the set point of 270 °F.

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

The thermochromatic paper inside the pack will indicate if 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 or partial color change indicates an unsuccessful test cycle. Click here to find out the common causes of failed Bowie-Dick test cycles.

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

This test can be performed daily, or less frequently depending on the facility’s needs and SOP. If the Bowie-Dick test should fail, both the autoclave and the laboratory facility utilities should be checked. Any Prevac Cycle should not be used with any confidence until the test is repeated with a passing result.

Vacuum Leak Test

The Vacuum Leak Test is used to determine the air-tight integrity of a prevacuum autoclave’s chamber and plumbing system. This test exposes the autoclave’s plumbing and components to vacuum conditions and measures how much vacuum depth was lost over a given period of time. A typical Vacuum Leak Test Cycle will consist of three vacuum/pressure pulses followed by a 15 minute dwell period at deep vacuum. Upon completion of the cycle, a leak rate will be displayed on the autoclave’s control screen in units such as psia/min, kPa/min, mbar/min, or mmHG/min. The pass/fail criteria for a Vacuum Leak Test is ultimately determined by the specifications of the user, but industry standards call for an average leak rate of 1mmHG/min or less. The frequency of the test depends on the facility SOP and risk tolerance. Regularly performing a Vacuum Leak Test allows greater confidence in the integrity of the chamber and plumbing.

Laboratory autoclaves are calibrated upon installation, but it is important to periodically validate that they are operating properly. The Bowie-Dick test and Vacuum Leak test are simple operational checks that should be incorporated into the Standard Operating Procedures wherever prevacuum autoclaves are used.

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Association for the Advancement of Medical Instrumentation, ANSI/AAMI ST-8

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