This test is a steam penetration test, similar to the small load test and intended for daily use. This test is also used to identify a level of air removal or reduction in the level of non-condensable gases in the steam within the chamber sufficient to qualify the sterilisation process for a wide range of cannula, metal and textile products. A chemical indicator meeting the requirements of 11140-3 is placed in the centre of a standard test pack and a pass is identified from a uniform colour change to the indicator.

The standard test pack described, offers a challenge to the sterilisation process nominally the same as the challenge from the textile test pack described by Dr Bowie. Indicators conforming with 11140-4 can be used as an alternative to the standard test pack for conducting the Bowie and Dick steam penetration test.

ISO 17665:2024- Annex C.5

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NOTE - The rate of steam admission to the chamber during pressurisation to the sterilisation stage can have an influence on the measured thermometric values or the visible change taking place in the chemical indicator. Thus, if the rate of pressurisation is slow then any residual air pocket can be heated close to steam temperature leading to an inaccurate determination of temperature difference between the drain and the centre of the test pack.  Similarly, any residual air present can be sufficiently heated and humidified during slow rate of pressurisation to allow a visible change in a chemical indicator placed within the test pack.

Introduction

Sterilization is achieved by the rapid and even penetration of steam into all parts of the load and the maintenance of these conditions for the specified holding time. To ensure this, it is essential to remove air from the chamber and load, and to provide a steam supply which contains a minimal volume of non-condensable gases. Any residual air and non-condensable gases will become concentrated as a bubble in the load and inhibit steam penetration.

The Bowie-Dick test shows whether or not steam penetration of the test pack is even and rapid, and thus by implication that air or other non-condensable gases are not present. It does not confirm that the sterilization conditions in the load have been achieved.

Principle of the test

The test, as originally conceived and described in earlier editions of HTM 10 (Bowie, Kelsey and Thomson, 1963), is based on the use of a chemical indicator in the form of an adhesive tape stuck to a piece of suitable paper to form a St Andrew’s cross. This indicator paper is placed at the centre of a test pack of folded huckaback towels and then subjected to an operating cycle. The indicator tape shows a change of colour in response to a combination of time, temperature and moisture.

If no air is present in the chamber, steam will penetrate rapidly and completely, and the indicator will show a uniform colour change. If air is present, it will collect within the pack as a bubble. The indicator in the region of the bubble will be of a different colour than elsewhere on the paper, because of a lower temperature, lower moisture level or both.

The modern Bowie-Dick test uses a Class B chemical indicator conforming to BS EN 867: Part 3 contained within a standard test pack. The indicator is distributed over an A4 paper sheet in the form of a geometric pattern.

When used in conjunction with a standard test pack, Class B indicators are designed to show a failure either if, at the start of the holding time, the temperature at the centre of the test pack is 2°C or more below the temperature in the active chamber discharge; or if the indicator is exposed to insufficient moisture. Both conditions are usually caused by the presence of air or other non-condensable gases. Because of the tolerances necessary in the manufacture of chemical indicators, users should be aware that in order to detect a temperature difference of 2°C the indicator may show signs of failure with a smaller temperature difference.

Test Procedure

The Bowie-Dick test is normally preceded by a warm-up cycle. This cycle is necessary because the effectiveness of air removal may depend on all parts of the sterilizer being at working temperature. A satisfactory sterilizer may give a fail result if this is not done.

Remove the wrapping from a standard test pack and place the indicator paper in the sheet located nearest to the centre of the pack. Reassemble and secure the pack and replace the wrapping.

Place the test pack in the chamber with the bottom of the pack supported 100 to 200 mm above the centre of the chamber base.

Select the Bowie-Dick test cycle. Ensure that the holding time will not be longer than that specified in the table below. If this time is exceeded, the indicator may change in such a way as to make it difficult to detect the variations that would indicate a fail condition. Start the operating cycle.

During the holding time, note the reading on the cycle counter, the chamber temperature indicator and the chamber pressure indicator.

When the cycle is complete, remove the indicator paper from the test pack.

The test should be considered satisfactory if the following requirements are met:

• there is a uniform change throughout the indicator;
• the automatic controller indicates that a Bowie-Dick test cycle has just been completed.

It is important to compare the colour of the indicator at the corners of the paper with that at the centre so that any difference can be clearly seen. If there is any discernible difference the test should be recorded as failed, and the paper marked accordingly. A large area of unchanged indicator points to a gross failure.

An unsatisfactory test result indicates that the machine should not be used until the fault has been rectified. It is important to realise that if a sterilizer fails to pass the Bowie-Dick test it cannot be made safe simply by increasing the holding time until a uniform colour change is produced. A failed sterilizer is in urgent need of skilled attention.

Several factors may inhibit steam penetration and cause the test to fail.  Common causes of failure include the following:

a. an inefficient air removal stage;

b. an air leak during the air removal stage;

c. the presence of non-condensable gases in the steam supply.

A subsequent Thermometric Test Small Load will assist in diagnosing the cause of failure:

1. if the test reveals a temperature depression at the centre of the test pack, the problem is likely to be inefficient air removal or an air leak into the chamber. Air remaining in the centre of the test pack is inhibiting the penetration of steam and the correct temperature is not being attained. The sterilizer should not be returned to service until it has been subjected to a vacuum leak test and an air detector function test;
2. if the test fails to reveal a temperature depression, the problem is almost certainly air or other non-condensable gases in the steam supply. In this case the correct temperature is being attained but the steam is diluted, and insufficient moisture is present to change the indicator. The sterilizer should not be returned to service until the steam supply has been tested for the presence of non-condensable gases.

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