In validating the biological indicator, there is a need to calculate the survival /kill time .
So , while I’m searching, I have got to know about 2 equations that can we use to calculate the survival and kill time .
But , I have a doubt in that formula
Which , Kill time = log 10 ( population +4 ) x Dvalue **
** Survival time = log 10 ( population -2 ) x Dvalue

My question is why we have to add 4 for the kill time and substrate 2 for the survival time .

These values are set in the formulas by standards and pharmacopeias to make sure, as a manufacturer of biological indicators, that the BI will present growth or no growth based on SLR (Spore Log Reduction).

They act as a safety margin, to raise the probability that:

A BI will show growth if sterilization time is less than survival time: guaranteed at least a spore count of 100 survival spores (hence the -2 in the formula)

A BI will show no growth if sterilization time is greater than kill time: probability of 1/1,000 to have un survival spore (hence the +4 in the formula)

So it is just to have better assurance that BI will behave as expected, and let know the en user if it suits their intended use in their process. Any time between the survival time and the kill time will fall into an uncertainty range (or gray zone) where there is a considerable probability of having either growth or no growth on the BI.