The Principles of Medical Device Decontamination

There are chemical indicators (CIs) which consist of a mixture of reactive inks printed on a substrate which respond to defined characteristics of the process so that a visible change is observed after suitable exposure e.g. a simple colour change.

Physical Indicators (PIs)

Physical Indicators are the instruments fitted to the sterilizer which allow the control system to control the process and the user to observe and ensure a correct process has been carried out. Separate monitoring and control sensors and measurement chains must be used so that one acts as a cross check for the other. The monitoring sensors data is used for batch release. Physical Indicators usually respond to only one of the process variables and therefore an indicator for each is required (e.g. temperature).

Temperature Sensors

For steam sterilization processes temperature sensors are usually located in the chamber drain / discharge line and this is normally considered the reference measurement point from where the sterilization process is controlled, monitored and recorded. The sterilization process will be designed to ensure the reference measurement point is maintained at the sterilization temperature (e.g. 134 o C) and within the sterilization temperature band (e.g. 134 to 137 o C). A temperature sensor will also be included in the chamber space at a location defined by the sterilizer manufacturer. The temperature measured at this point provides evidence of chamber conditions and may be slightly higher than the drain but not more than 2 o C during the holding time. An overshoot in chamber temperature is allowed for a short period as the pressure within the chamber stabilises after steam admission, but equilibrium with the chamber drain must be established within a short period.

Timers

Timers are an integral part of the control system which in a modern sterilizer will be via a programmable logic controller (PLC). Failure or errors in the measurement of process time rarely occurs and would be indicative of a major fault in the control system if it were to happen. This is the purpose of secondary independent monitoring systems which allow cross checks to be made.

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