Why we are considering 6 log reduction for autoclave validation

Dear team,
May i know the reason for the considering 6 log reduction for autoclave validation as per which guidelines we are following that phenomenon.

Please give me your valuable suggestions and answers.

Ratna raju

in the pharmaceutical industry, in order to define a unit as sterile one
must be able to certify, on a statistical basis related to the conditions of preparation and
sterilization of that specific product and of that specific batch, that less than one unit in
a million is exposed to the risk of not being sterile. The probability of finding a nonsterile
unit (PNSU = Probability of Non Sterile Unit, or SAL) must therefore be smaller (as
mathematical value) than 10−6.

*This means that at the end of each minute at 121°C in moist-heat conditions the
number of microorganisms reduces to one tenth of the number at the beginning
of that minute.
*Therefore, if a unit is kept at 121°C in moist-heat conditions for 3 minutes, the
number of microorganisms contained therein is reduced to one thousandth (1/10
x 1/10 x 1/10 = 1/1000) of the initial number.
F0 - A technical note – Doc. 352178v2 - Copyright © 2014 Fedegari Group 28

  • If the initial bacterial load of a batch of units being sterilized is on the average
    1000 (i.e. 1000 microorganisms per vial or bottle), after 3 minutes of treatment in
    moist-heat conditions at 121°C it is reduced on the average to 1.
    *After a further minute of sterilization (4 minutes altogether) this reasoning leads
    one to the conclusion that the load has dropped to 1/10, i.e. 0.1. However, this
    must not be understood to mean that at this point each unit contains one tenth of
    a microorganism (in which case the units would be sterile…) but must be taken to
    mean that there is a probability that 1/10 of the units are still contaminated.
  • After 9 minutes of treatment at 121°C in moist-heat conditions, the bacterial
    load of the batch at issue is reduced, on the average, to 1/1,000,000. The
    probability of still having a contaminated unit in that batch is therefore 1 in
    *This is the minimum assurance of sterilization which must be achieved in the
    pharmaceutical field, though a greater assurance, for example 10−9, i.e. 1 in 1
    billion, is often sought.
    *This assurance is expressed as PNSU (or SAL): Probability of Non Sterile Unit.
    PNSU =10−6 or SAL =10−6 means that the probability of finding a non-sterile unit in
    a batch is 1 in 1 million.
    *The “overkill”, i.e. “over-sterilization”, approach is generally used when a
    sterilization process for heat-resistant products is validated.