DECONTAMINATION
Ensuring effective testing: five ways to ‘fail’
As the steam steriliser is a critical part of decontamination, verification of the machine’s ability to penetrate steam into a load is vital to ensure patient safety. Emma Barrett and Richard Bancroft offer some advice on deciding which steam penetration verification test is right for your facility.
Throughout the UK and the rest of the world, fast-paced operating theatres rely on the quick and thorough decontamination of the critical instruments required to perform life- changing and often lifesaving procedures, and, since its invention, nearly 140 years ago, the steam steriliser has remained a critical part of the decontamination process. The amount of energy needed to convert boiling water to steam is much larger than the energy required to simply heat water. In order to turn liquid water into gaseous steam, we must apply enough energy to overcome the heat of evaporation (sometimes known as the enthalpy of vaporisation). The amount of energy per molecule to convert boiling water to steam is seven times higher than the amount of energy per molecule to boil water; steam at 100 °C has seven times more energy than boiling water.
It is this energy that is available to heat the load in the steriliser extremely rapidly, as the heat of evaporation is released, turning the water back into the lower energy level, but still at 100 °C. The other important factor in this process is the volume occupied by water and by steam; when in the liquid phase i.e. water, 1 kg will occupy 1 litre of volume. When converted to steam, 1 kg will occupy 1,672 litres of volume at 100 °C (this volume is temperature-specific, but steam tables give precise values over a range of temperature values).
It is not just about the energy though. In order to condense onto loads and heat them,
steam must be able to come into contact with the entire surface. Steam is able to penetrate into porous materials; as these materials will have a cooler temperature than the steam, it will condense into water as it hits the cooler surface, creating a negative pressure as the steam turns into water and its volume reduces, which draws more steam to this cooler area. Steam is constantly drawn towards the objects in the steriliser until they reach the same temperature as the steam. Unfortunately, if air is also present in
While we aim for a Bowie Dick pass result, it is the fail result that is most important – an indicator that never fails has failed as an indicator.
AUGUST 2020
the steriliser, this air will also be carried alongside the steam into and around the load. Steam’s ability to penetrate dense materials and rapidly heat surfaces, destroying microorganisms, will be significantly compromised by the presence of this air. Air in a steam steriliser is termed a non-condensable gas, precisely because, unlike steam, it will not condense onto loads. This build-up of air is sufficient to compromise the ability of steam to destroy microorganisms.
The introduction of verification for steam sterilisation
Since the creation of the first steam sterilisers in the 1880s, we have understood that residual air within the chamber stops complete penetration of the load by steam, preventing the successful sterilisation needed to keep patients safe. It would take nearly 80 years before this issue was sufficiently understood and the work of Bowie, Kelsey and Thompson would eventually lead to the introduction of the first Bowie Dick test in the 1960s.
The Bowie Dick test pack was designed to be processed alone in the steam steriliser to simulate the way the steam would affect an actual steriliser load. A single load in an otherwise empty steriliser chamber is actually the worst-case scenario for air removal; any air present in the steriliser will be concentrated into the single load, aiding its detection by a chemical indicator placed inside the pack that changes colour to indicate it had been exposed to steam. A successful Bowie Dick test assures the user that air had been completely removed from the steriliser chamber and as a result, that the porous loads had been fully penetrated with steam. Finally, daily verification of the steriliser’s ability to penetrate steam into a load was available, and to this day, millions of Bowie Dick tests
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