F0 and holding time


I’m gonna to validate one lab autoclave, the cycle are already programed (121 C, 20 min) , I’ll put the TC inside the loads and calculate the F0 during the holding time.

My question is for the acceptance criteria, can I put that the calculated F0 is no less than the actual holding time? what is the acceptable range normally (it is possible the calculated value is a little bit less than the actual holding time).

thanks for explanation!

acceptance criteria depend of the load and directive you follow.

If we talk about liquid load, then acceptance criteria can be Fo greater than 15 minutes and inactivation of ampules of b.stearothermophilus (overkill cycle PDA Techical Report).

If we are talking about porous load, according to PDA overkill cycle Fo greater than 15 and inactivation of b.stearothermophilus.

But, EN 285 says that equlibrium time must be less than 30 seconds and hold time at least 15 minutes. Then, this hold time can be shorter than yours 20 minutes. You can not compare Fo with hold time cause Fo is cumulative value that depends of temperatue and time, so you can have Fo higher than 15 for cycle less than 15 minutes.

So, if your holding time is greater than 15 minutes and your equilibrium time is shorter than 30 second it is obvious that Fo value will be greater than 15 minutes.

This can be your acceptance criteria: holding time is greater than 15 minutes and equilibrium time is shorter than 30 second and inactivation of BI b.stearothermophilus and you have all conditions for effective sterilization.

Thanks so much for reply!

As to the 15 minutes for liquid and porous load, which part can I find in the PDA report, the original version or any updated one?

PDA Techical report (update version):

For overkill cycle see section

For porous load / hard goods see section 4.2.1
If air removal cycle and if gravity cycle see section

For liquid load see 4.2.2
If overpressure is requested see section 4.3.2 ( and and if there are no request for overpressure then see section 4.3

very helpful!

thanks a lot.

Hi Zellko,

As per your accaptance criteria i would like to know the

1.With 15 min hold time you may not achieve 12 log reduction or overkill approach
2.Equilibration time for liquid load may not applicable
And also if the hold time is 20 min or 30 min (porous load) as per the initial validation same Fo value can set for load with nullification of BI,s and for liquid load lag to be considered.


  1. Log reduction can not be connected with hold time, because we dont know is temperature in hold time 121, 122 or maybe 123oC. We connect log reduction with Fo value. Fo value 15 minutes is overkill cycle. Why? When we are talking about log reduction, we know that it is calculated with equation Fo/D. For overkill cycle it is enough Fo greater than 12, for D value 1. It is worst case cause in production cause you do not have b. sterothermophilus in products of course. Most resistant is I think Clostridium Sporogenes with D value 0.8. So, we have more than 12 log with Fo greter than 15 minutes. (I am talking about Fo 15 not 12 as in PDA cause it is from EuPh so I take this as worst case criteria).

  2. Equilibration time for liquid load is not applicable because porous load and liquid load are two different aproaches. For porous load you must be sure that surface is with direct contact with steam not with trapped air. So, equilibration time is telling us is it hot air or steam. For liquid load, we have transfer of heat so, it is enough that we have in our case at least 15 minutes at 121oC (EuPh).

Duration of cycle for liquid load is something you must validate, and have to consider lot of factors(cold spots, heat lability of product and others) but if you prove that you have in every container 121oC 15 minutes it is really ok.

For porous load according to EN 285 validate cycle that your equilibrium time is 15/30 seconds and after that time hold time is enough 15 minutes.

Hi All,

Talking about D-value, is it different from strain to strain of B. Stearothermophilius? I saw most put at 1.6min, but our stored B. Sterothermophilus (ampouple) give a D-value of 2.3min, can anybody explain this? which should we use?


Yes, D value is different from lot to lot of BI for the same BIs. In your case one lot has D value 1.6 and another 2.3. Every BI B. Stearothermophilius with D value greater than 1.5 minutes is acceptable. The difference between lots is that they are different parent crops. They were grown at different times and have slightly different D-values. You can use both, depend of your cycle.

Here arise the question, if I use the strain with D-value of 2.3, can I still be able to use the formular, F0=10 (T-121)/10, where Z=10, how can I explain the F0, ie if I get 20 min, it means the sterilization effect equals to 20 min of 121 C to kill a strain with ( D=? Z=10)?

is it the same case that it need 10 C change to cause 10 log reduction of D 2.3min (at 121C) and D 1.5 (at 121C) ? in the formular, the Z is based on what strain (D-value at 121C)?

You calculate Fo with formular F0=10 (T-121)/10, where z=10 and it is only formular for Fo calculation. If you have Fo 20 minutes it is like 20 minutes at 121oC or higher temp and shorter time. And there is no connection with D values of BI. Fo values says what ammount of heat you deliever to load and it is used to compare different tem and time. If you have D value of 2.3 minutes of BI and your Fo is 20 minutes you have 20/2.3=8.7 log red which exceed your 6log of BI.
Fo is physical value and you can have Fbio as biological value.

I belive you are very experienced, I have several other related questions about autoclave temperature test and I’d appreciate it very much if you can help:

is there any standard acceptance criteria of :

  1. Temperature variance of one TC during the holding time
  2. Temperature difference between different TC during the holding time
  3. Temperature difference between TCs and autoclave set point (say 121C for example)
  4. Time difference of TC’s holding time and set point (30 min for example)

In the literature, I saw different acceptance criteria with different people, is there any reference I can justify my acceptance criteria?

Hi Hanks,

according to EN285 (HTM2010),

  1. Temperature variance of one TC during the holding time - the measured temperatures do not fluctuate by more than 1 °C
  2. Temperature difference between different TC during the holding time - the measured temperatures do not differ from one another by more than 2°C;
  3. Temperature difference between TCs and autoclave set point (say 121C for example) - the indicated and recorded chamber temperatures are within 2 °C of the temperature measured in the active chamber discharge
  4. Time difference of TC’s holding time and set point (30 min for example) - holding time must be greater than 15 minutes and equlibrium time less than 30 sec so no metter set point is.

Why there are so different aceptance criteria?

It depends from regulations, US (PDA Techical report) likes Fo values like ammount of heat you deliever to the load, not temperature and likes BIs.

On the other hand, GB standard (EN285) likes the conditions you have in your sterilizer and if you have steam at boundary phase on every surface, not air, your sterilization is efficient.

What is failing of PDA approach? You are not sure that if you have Fo values greater than 15 is this from steam or from air? Yes, you put BIs but sometimes it can failed. On the other hand, EN285 put accent on sterilization conditions - holding time, eq time…

We are audit from EU and GB and USA inspections so we must have all criteria - eq time, Fo greater than 15 (at the same time hpld time is greater than 15) and BI inactivation.

I learned a lot and it is nice to talk to you, thanks a lot for your sharing!