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What is Biological Indicator Survival & Kill time verification?

Survival time is the exposure time required in order for all BI’s of a defined population to retain viable spores. 

Kill time is the exposure time required to achieve inactivation of a population of Biological indicators to the extent that the probability of a viable biological indicator being 1 in 10,000. This can be calculated theoretically using the initial Biological Indicator Population and the D-value at a given temperature.


Why do we do it?

The purpose of this step is to ensure that the biological indicators possess sufficient resistance to validate sterilization of a process and will not display too much resistance resulting in unexpected growth.


How do we help assure compliance?

Theoretical survival and kill times are calculated using the following formulae:

Theoretically Log10(Population) x (D-value) = 1 spore per Biological indicator

Log10(Population+4) x (D-value) = Theoretical Kill time

The “+4” adds 4 further log reductions beyond a spore count of 1 spore per BI.

Log10(population-2) x (D-value) = Theoretical Survival time

The “-2” removes 2 log reductions before a spore count of 1 is achieve, leaving 100 spores per BI. This gives a high chance of growth on all BIs.

The verification is performed by adjusting the length of BI exposure. The Kill time is the shortest cycle that will achieve full kill of the BIs. The Survival Time is the longest cycle that will achieve growth on all BIs.

For further information and advice on Biological Indicators please browse our knowledge hub below, or contact us about your analytical testing requirements.

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