I would like to submit the following question:
As quality engineer for a medical company in disposables I was asked to do a bit of research into sterilisation. It might be a simple question concerning the population reduction: what does the figure "6 log10" actually stand for? That will probably also answer my second question which is: what is better: 4 log 10 or 6 log 10 reduction?

Best regards,
Marc Miesyerus

Dear Marc,

Question Q00137 gives you an answer (EN 556: "Requirements for medical devices to be labelled sterile "). In the latest issue of "Central Service" 4/02, p248, Sterilization Basics, Peter Hooper describes: "What is sterilization".

Good luck,
Wim Renders

Hi Marc

The short answer to your questions is: D-Value is the time in minutes required - at a certain temperature - to reduce the number of viable microorganisms with a factor 10.
I know this is not the answer you are looking for.
To go into details the issue is to complex to be answered here. Look for Literature on Sterilization.
In addition to Wim Renders advise put your attention also to the recommended books on our Website: Education - Book Recommendations. You will find an answer to your question in detail in the Book "Sterilization of Medical Supplies by Steam" written by Jan Huys. The chapter "Reducing the bioburden" with subtitles like "the Death rate of microorganisms and the Decimal reduction time; D-Value, is just one of the thirteen excellent written chapters. It is a textbook that can be used by anyone interested in sterilization.

Kind regards
Josy Holdener

Marc this is the answer to your question

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 * 10x6 (one million) and we reduce the population to 1 x 10x0 (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 10-6 or one survivor in one million.

So marc this is overkill and a serious deterrent to any offending pathogen.

Hugh O'Connor