On Italian norm UNI EN 556 "Sterilization of medical devices - Requirements for medical devices to be labelled Sterile" there is the definition of "STERILE" medical device. The theorical probability of 1x 10 to 6 is referred to the microrganism in each medical device or contaminated medical device on 1 million of medical devices produced?

Alberto Franceschini
Sterilization production center
Ferrara - Italy

Alberto

I know, EN556 can be very confusing and on first parsing the draft document. The impression and understanding is that its OK for one little infectious pathogen to survive and reproduce in its nasty but propagating environment. But that is not the case, you see sterilization processes are not instantaneous unless your in possession and zap them with a nuclear capacity. The time taken to kill a population of bacteria depends on temperature and the no of organisms to be killed. The killing process has to be so efficient that not only is there no chance of a single survivor at the end of the process but that the chance of survival is reduced to a fraction.
So we design a process to be powerful enough to deal with the most resistant forms of bacteria (spores) and then ensure that the chance of survival are less than one in a million. This is the level defined in EN566 European standard.
The concept assumes an initial population of 1 million miros or 1 * 10e6 (one million) and we reduce the population to 1 x 10e0 (or 1). To extrapolate are formula and to design an effective sterilization process we must then reduce the one nasty little survivor to a fraction of reproducible viability and we are now into the realms of probabilities.
One in ten, one in a hundred, one in a thousand, one in a million , now the definition in EN556 requires a SAL of 1 x 10e-6 or one survivor in one million.
So alberto this is overkill and a serious deterrent to any offending pathogen.

Hugh O'Connor

Dear Alberto,

I just want to add two definitions from "Sterilization of medical supplies by steam" by Jan Huys:
"A product is considered sterile, if the chance that there are viable microorganisms on the product is equal to or less than 1 to a million". Another way to say the same thing is: "A product is considered to be sterile if on a total number of 1.000.000 products that were sterilised, there is 1 or less than 1 surviving microorganism".

Best regards,
Wim Renders