# Steam Sterilization Lethality (Fo) Calculation

As i saw in a document, Fo is calculated by considering D value in equations. For example the equation is written as Fo=1/D*(10^T-121.11/z)*∆t.

But in some documets the equation doesn’t have D value. The usage of D value creates some questions, such as;

1)Does Fo have a unit? (minute?)
2)When i put D value in equation, Fo value is effecting very much and so the kill time change dramatically to reach the determined Fo value.

For example, if i use D value in equation and take D value as 2 min, Fo value becomes the half of the Fo value when D value is 1 min.

But most of the documents say that there is no D value in Fo calculation equation.

Up to today, i had been entering the D value to validator, but as i heard from other pharmaceutical companies they neglects the D value and let the validator calculate Fo without any D value.

The BI strips i use in autoclave qualifications, has 2 min D value. In such situation, should i enter the D value to validator or just let the validator works without a D value?

Fo=1/D*(10^T-121.11/z)*∆t.
It is assumed that Do value for 121oC is 1 minute so, it is the same equation.

1)Does Fo have a unit? (minute?)

Yes, Fo 1 minute is one minute at 121oC (unit is minutes)

2)When i put D value in equation, Fo value is effecting very much and so the kill time change dramatically to reach the determined Fo value.

To compare Fo values at different temperatures it is assumed that D is 1 minute at 121oC and z is 10oC

You dont put D value in kaye validator, when you talk about Fo it is physical parameter but if we talk about D value of Bi it is byological parameter.

Still there is an issue confusing my mind. As it is written on the certificate of BI i use in qualification test, D=1.9 minand Z=9oC.

Therefore, i think like this way; i should enter this values to validator and during the qualification test when i reach Fo=15 then i can assume that bacterias in BI are destroyed now.

By the way, the kill time of the BI is written as 20 min. This means that, Fo=12 for this BI. 12 min exposure time at 121oC is enough to sterilize.

I think, i should make the run 20 min at 121oC (that is not possible) or i should resume the run until Fo=12 (for the mentioned BI).

We all use Fo values in our test because it is not possible to hold the autoclave at 121oC. And so, the exposure time is accepted longer than the kill time.

The acception of D=1 min and Z=10oC, make me think that i m doing wrong. By the way, if i use D value even 1 min, Fo become unitless.

First, dont mix Fo values with BIs that you use. Fo value is physical value and it compare different cycles.

Fo compare ammount of heat that you deliver to your load. And same Fo can be for your cycle at 15 minutes at 121oC and shorter cycle at 124oC with the same steam conditions.

If your cycle has Fo 15 it is 15 no metter which BI you use and no metter what is D value. You dont enter your D value in validator because there is no logic to have SAL 10at -6 for B Steraothermophilus because you dont have them in production.

For EuPh overkill cycle is 15 minutes at 121oC and this is worst case with enough safety margine. D value 1 minute is worst case that you can have in production so, Fo of 12 minutes give you SLR of 12 or SAL 10at -6. Fo is cumulative value and validator calculate it with equation Fo=1/*(10^T-121.11/z)*∆t and z=10.

Can any please help me to undrestand, what is the significance of D value in sterilization validation process.
Do we need to calculate F0 value of Biological Indicator also using D value to compare with Process F0 value.
Process F0 value calculation, as far as i understand, uses Z value (Standard and constant value) and Process temperature. Then what is the use of D value?

Fbio is primarily used in Cycle Development in conjuction with Fphys. If you are using a validator on your PQ, keep the defaults lethality setting of D=1 and Z=10. I made the mistake of replacing this with my BI info in the past. You can determine the lethality value of your BI by D121.1 X (log A - log B) where A is the initial population (10^6) and B is the required minimum sterility level (SAL 10^-6 or 12 log kill). But for PQ purposes, I would stick with your default Fphys and let the validator do the computation for you. It is required to have a F0=12min. Hence, a 15min cycle at 121.1 gives you a pretty good margin.