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Decontamination Process Validation
The Concept of A0

Understanding Thermal Disinfection

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What is A0?

Engineers are increasingly asked to calculate the A0 value of a thermal disinfection cycle to include in their test reports. Although, TQSoft and IPReports will generate this automaticall for you, it's worth looking at what it is, and what it actually means (beyond A0 = ∑10(T-80)/z ∆t).

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When using TQSoft and IPReports Validation Software the A0 value can be imported directly into the report at the touch of a button.

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A0 can be defined as the amount of energy put into the disinfection process in order to achieve the desired level of killing micro-organisms, as an expression of temperature over time.

If you consider a graph of the temperature during a cycle, this can also be seen as an expression of the area under the curve during the period where disinfection is taking place (generally 65°C and above). The A0 is therefore the time taken to achieve the required log reduction in micro-organisms and can be viewed as a measurement of how much the number present has been reduced.

Note that the A0 calculation should also be stopped after disinfection ends so that high temperatures during drying (when no disinfection is occurring) are not included.

 

A0 Chart

A0 is usually a European concept, with the number following A0 representing the number of seconds at 80°C that is required to achieve this amount of micro-organism reduction.

This is a logarithmic scale, so for every 10 degrees increase in temperature you require a tenth of the time (see the table below).

The standard requirement for surgical instruments is an A0 of 600 (equating to 10 minutes at 80°C), while Human Waste containers require an A0 of 60. there is also provision for an A0 value of 3000, and this may be asked for by the End User, but there is no standard application that requires this.

Applying different temperature and time combinations, A) 600 could be achieved with the following:

Temperature °C

93°C
90°C
80°C

Time-Minutes

0.5
1
10

Time-Seconds

30
60
600

It should also be noted that is you have an disinfection temperature of 80°C for 10 minutes (or 90°C for 1 minute), then by definition you will attain an A0 value of 600- or usually more, as the machine is almost certainly going to exceed both the 80 degree and 10 minute values.

In the UK as a rule the standard is that a cycle has failed if it drops below 80°C (or 90°C) during the hold time, even if the A0 value would have been above 600, so in many ways A0 becomes moot, unless there is a requirement for an A0 of 3000.

The A0 lethality calculation is identical to F0 for autoclaves – more information on the calculation can be found here >