Understanding steam sterilisation failures

Staff working in decontamination services will ensure that reusable medical devices, such as endoscopes and other surgical instruments and equipment are cleaned, sterilised, and repackaged to high standards. With sterilisation (or sometimes decontamination), one of the commonly used items is the autoclave. The autoclave is one of the longest-established sterilisation technologies using a method of sterilisation involving moist heat (achieved using steam).1

Well-designed and suitably qualified autoclaves operate well and consistently. However, autoclaves can periodically go wrong, such as when improperly maintained, or they can fail at the point of revalidation. This article presents an overview of autoclave operations and considers those things that can go wrong and affect autoclave operations.

An autoclave is, crudely, a pressure-cooker, based on the manipulation of pressure to raise the boiling point of water. Water boils at 100°C, at atmospheric pressure; whereas at lower pressures it boils at lower temperatures and at higher pressure it will boil at a higher temperature. For example, at a steam over-pressure of one bar (or 100,000 Pascals) water boils at approximately 121°C. This allows the autoclave to produce temperatures above those that can ordinarily be achieved. Then, if operated for sufficient time, and with the correct conditions, such temperatures can destroy bacterial endospores. The standard sterilising temperature in steam autoclaves is 121°C, but lower (such as 116°C) and higher (for example. 134°C) temperatures are also used for certain cycles. For the sterilisation of microbiological culture media, for example, a lower temperature is required to avoid physical damage.