Manuale GW3060 - User
Versione 22.10.08
31
7.3
THE THERMAL DISINFECTION PROCESS
Over the last few years, many efforts have been made to improve the decontamination of instruments and
glassware: specific standards for both the process and techniques used have been issued and have defined the
state-of-the-art. All this has provided the end-user with quality and safety advantages.
We would now like to go on to describe the
A
0
concept (see Tab. 1 on the next page) defined by EN ISO 15883,
which explains the meaning of the temperature and time values indicated in the thermal disinfection programs.
Value
0
A
indicates the degree of thermal disinfection of the corresponding program: thermal disinfection is all
the more efficacious and suitable for particular instruments the higher its value is.
Research has been conducted according to which, by conducting washing cycles that comply with these 2
parameters, i.e. temperature @93°C and holding time @10 min., one obtains a high degree of microbiological
disinfection in the treated materials, such as to inactivate the majority of the bacteria (with the exception of
spores) and heat-resistant viruses such as the HIB viruses of hepatitis A and B as well as the HIV virus
responsible for AIDS.
Thermal disinfection processes that provide a good degree of disinfection can also be carried out at lower
temperatures and with shorter holding times. The temperature/holding time combination depends on which type
of disinfection is required.
A
stands for the equivalent time in seconds required to produce a given disinfection effect at a temperature of
80°C (taken as a reference). If
D
is the time required to reduce the bioburden on the instrument treated by a
factor 10 (thus with a 90% “lethality” degree) at a particular temperature, value
z
can be expressed as the
temperature increase (in K degrees) required to reduce
D
(in boldface) by a factor 10. In view of this,
A
0
stands
for the value of A when the temperature is 80°C and value
z
equals 10.
In short, the value of
A
0
expresses the degree of “lethality” in seconds at a temperature of 80°C, released by the
thermal disinfection process to the device under treatment, with reference to microorganisms that possess a
z
value of 10 (a characteristic valid for many microorganisms). The
A
0
value that must be obtained depends on
both the type and number of microorganisms on the contaminated medical device and on its successive use.
According to EN ISO 15883 and the recommendations of the Robert Kock Institute (European authority on the
subject), an
A
0
of 600 is considered as the minimum standard for non-critical medical devices, i.e. for those that
only come into contact with uninjured skin. A further condition required is that microbic contamination must
only be slight and there must be no heat-resistant pathogens present. An
A
0
value of 600 can be obtained by
maintaining a temperature of 80°C for 10 minutes or 90°C for 1 minute or again, 70°C for 100 minutes (lowerer
temperature are not recommended).
If instruments are contaminated with heat-resistant viruses, such as those of hepatitis B, the value of
A
0
must be
at least 3000. This can be obtained by maintaining a temperature of 90°C for 5 minutes.
An
A
0
value of 3000 is considered the minimum value to apply to all medical devices considered to be critical.
Programs that include thermal disinfection have therefore been designed to offer the following
A
0
values:
TEMPERATURE AND
TIME
A
0
90°C for 1 min
600
90°C for 5 min
3000
93°C for 5 min
6000
93°C for 10 min
12000
For the sake of convenience, the formula for calculating
A
0
is given below
⎟
⎠
⎞
⎜
⎝
⎛ −
•
=
10
80
0
10
T
A
τ
where:
