I have been using F0 bio = D 121 (log A – log (2.3 log (n/q)))

where

A =initial spore population

D121 =D-value

n =number of BI exposed

q = n-1

The above formula was being used at a client company. However I referenced PDA Validation of Moist heat sterilization process and the formula specified was

F bio = DT *LR

where DT = D value of the BI

LR = Log N0 - Log NF

Hello, and welcome!

The PDA formula you referenced is correct, and is consistent with pharmacopeias.

The first formula you mentioned:

F0 bio = D 121 (log A – log (2.3 log (n/q)))

comes from a formula used to empirically calculate the resistance (Dvalue) of a micoorganism using Stumbo-Cochran-Murphy methodology (included in pharmacopeias and ISO-11138).

What is your client using that result for? Is it using it to confirm FBio in a validation cycle? If that is the case… It will not be correct, since that formula requires a fraction negative of BIs (this is because the metod is to empirically calculate Dvalue based on fraction negative results). And your client would not be able to calculate: log (n/q) since you cannot calculate a log of 0. Additionally, the calculation would be for an entire cycle, and could not be calculated for each product or material (individual BI).

That is why, for validation studies, the use of biological indicators is widely used, following different methods stated by pharmacopeias and other guidance documents like PDA: overkill approach, combined biological indicator bioburden, or bioburden specific approach.

I hope this helps. Let me know if you need me to elaborate further.

Thank you for your response. Yes client was using the F0 bio = D 121 (log A – log (2.3 log (n/q))) formula to confirm FBio in their autoclave PQ validation protocols. The requirement specified is F0 bio >= 12. I argued about this when I started work for a project but now the issue has come to light as we have an an on going audit prep.

we are using overkill approach with set point at 123C for all cycles on the autoclaves, confirming three things for a SAL of 10 ^-6.

- penetration sensors lethality >=22 minutes towards end of exposure period
- BI organism kill
- F bio = DT * (Log N0 - Log NF) >= 12

Can you please confirm if the following info is correct when using F bio = DT *LR

DT = D value of the BI

LR = Log N0 - Log NF

N0 = initial population of BI

NF= Final population of BI

is NF assumed as 10^-6 for a SLA of 10^-6 ?

Thanks again.

Yes, it is correct; however, if you want your client to use that formula to calculate FBio of a cycle using BIs; then you cannot assume that the NF is 10^-6. That is only used to design a cycle, to calcualte the parameters of the cycle, like time and temperature required to meet process lethality (F0).

The problem is that when a BI is inactivated, its population is 0, and you cannot calculate: log (0). This is probably why your client used that other equation, but as I said earlier, it is misused.

If the BI was designed to meet overkill criteria, its inactivation is the confirmation of biological lethality, and no further Fbio calcualtion is expected, that is why it is very important to make sure that the BI challenge meets overkill requirements. G.Stearothermophilus is widely used as a BI challenge microorganism, with a high population (10^5 , 10^6 CFU/BI, and a D121 value of at least 1.5 min).

We have been using BI that meet the USP <1035> requirements. I gather that if we use BI that meet the overkill requirements, Fbio calculation is not required.