How Bowie Dick Test could save your life?

Introduction:

Steam Heat Sterilizer is an equipment used for sterilizing articles / products / aids / medical equipment for destroying viable microorganisms by moist sterilization. 

Bowie Dick test could assess inadequate vacuum, meaning inadequate air removal from the sterilizer chamber or a leaky door gasket that would also allow for residual air in the sterilizer chamber. This test evaluates the performance of the sterilizer used in Pharmaceuticals / laboratories or in hospitals. Failed BD test would indicate that the sterilization inside the chamber is not uniform. The articles sterilized and used for an surgical operation is not adequate or the product is not sterilized completely. This will impact the patient’s life by usage of an article / product which is not sterilized adequately. The failed Bowie Dick test would indicate that the retention of air due to an inefficient air removal stage or the presence of an air leak during the air removal stage. If such articles are sterilized in those sterilizers (which are not tested for Bowie Dick test) are used for the surgeries or drug manufacturing could lead to infections to the patients. So, this Bowie Dick test becomes highly relevant in Hospitals, laboratories and Pharmaceutical industries in the day to day operations.

Bowie Dick test 

The Bowie – Dick test is named after its developers.  J.H. Bowie and J. Dick. Originally, this test was used to evaluate the vacuum phase of single deep vacuum sterilizers, called high vacuum sterilizers.  It was a diagnostic test of the sterilizer’s vacuum system.   

Objective of the Bowie Dick test 

The air removal test is used to evaluate the efficacy of air removal during the pre-vacuum phase of a sterilization cycle. 

Failure of the Bowie Dick test 

Retention of air due to an inefficient air removal stage or the presence of an air leak during the air removal stage are circumstances that can lead to failure of the test. 

Causes for failure of the Bowie – Dick Test 

This residual air prevents thorough steam penetration into the chamber load during the steam exposure phase of the sterilization cycle. 

The Bowie-Dick Test procedure is the same for both qualification and routine testing. 

1. Warm-up cycle. Run a warm cycle prior to the BD test to avoid false positive test result. 

2. Test pack. Place the BD test pack near to the worst case location, over the drain in an empty chamber. The drain is where the vacuum is drawn, and any residual air in the chamber will naturally migrate to the test pack. 

3. Cycle run: Run the sterilizer as per the Bowie-Dick test pack’s instructions. 

4. Open the test pack and interpret results: After the cycle completion, take out the BD test pack and check for the colour change in the indicator sheet for interpretation. 

The Actual Test and the principle 

This diagnostic test utilized a sheet of paper to which were applied strips of autoclave tape in the form of a St. Andrews cross. The autoclave tape contained chemical indicator lines. The sheet of paper was located in the center of folded huckaback towels made from flax.  The assembled test pack was run in an empty chamber, positioned over the drain. 

Bowie Dick test – setup 

A typical Bowie Dick test being setup using a pack of towels 

Placement of the 16 – towel biological indicator test pack for installation testing 

A porous mass is simply a large group of microscopic cells

When vacuum is applied and steam enters the porous mass

Steam penetrates the pack and pushes the air from one cell to the next, closer and closer to the centre of the pack. 

As steam penetrates, the steam condenses when it touches the cooler air inside the pack. 

It thereby forms a somewhat spherical condensation boundary, which gets smaller and smaller as the pressure builds in the sterilizer. This condensation boundary contains an “air bubble” which interferes with penetration of steam and achievement of sterilization.

When the steam valve is closed and the drain valve is opened, the pressure drops quickly.  The air expands quickly as the pressure drops but moisture from the condensation boundary remains behind for a short time in the absorbent cellulose of the porous mass. 

When the pressure is sufficiently low, the moisture from the condensation boundary evaporates and pushes out any air, which is in front of it.  This leaves less air in the pack when the next steam pulse is added.  The condensation boundary and air bubble is smaller after the second pulse and smaller still after the third pulse and so on until all of the air is gone. 

Upon completion of the cycle, the sheet containing the autoclave tape is removed from the pack and observed for the uniformity of the indicator ink lines on the tape.

A non-uniform or lighter ink line in the center of the sheet (at the intersection of the strips of tape) would indicate a lack of steam penetration to the center of the test pack. 

Bowie Dick test failure reasons:

• Bowie Dick test pack preparation. 
• Sterilizer control calibration. 
• Facility utilities and steam quality. 
• Door gasket / air leak. 
• Steam supply issues. 
• Steam trap or safety valve failures.

Developments:

1 Numerous changes have taken place in test pack construction, pack design, and sterilizer cycle characteristics since Bowie and Dick published their findings in 1963. 

2. Towels used in the standard test pack are derived from cotton rather than flax.  An indicator sheet, called in “air removal indicator” in this standard, has replaced the autoclave tape. 

 3. The chemical indicator ink is printed on these sheets in some design or pattern that may cover up to 50% of the total surface area of the sheet.

4. Frequently, these are disposable test packs that have been shown to perform equivalent to the standard test pack containing an indicator sheet.

5. Further, pre- vacuum steam sterilizer being marketed today does not use single deep vacuum phases; they use pulsed vacuum cycles.  In these sterilizers, the depth of vacuum per pulse is not as great as in the older high vacuum sterilizers.  

 6. In these sterilizers, the depth of vacuum per pulse is not as great as in the older high vacuum sterilizers. 

“Sometimes we tend to ignore the smaller things which gets unnoticed. When we dive deep-in, we realize that those smaller things are important, like a small crack in a ship can sink the whole ship too..”

Muthu

Note-The images given for representation in this blog are taken from Google Images. Many thanks for Google.