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F0 Value, D Value and Z Value Calculations


Calculation of D - Value, Z - Value and F0 - Value for sterilization in autoclave used for microbiology analysis as per USP and BP.

F0-Value :

F0-Value at a particular temperature other than 121ºC, is the time in minutes required to provide the lethality equivalent to that provided at 121ºC for a stated time. (As per USP 2008)
D value, F0 value and Z Value CalculationF0-Value of a saturated steam sterilization process is the lethality expressed in terms of equivalent time in minutes at a temperature of 121ºC delivered by the process to the product in its final container with reference to microorganisms possessing a Z-Value of 10. (As per BP 2009)  
In general for aqueous preparations a microbiologically – Validated steam sterilization process that delivers, in total ( including the heating up & cooling down phases of the sterilization cycle ), an Fo-Value of Not less than 8 to every container in the autoclave load is considered satisfactory. (IP 2007)

Related: Importance and Determination of F0 Value in Sterilization

Determination of D Value and Z Value in Microbiology:

D-Value (Decimal Reduction Value) : 

It is time required at temperature T to reduce a specific microbial population by 90% or by a factor of 10.

Z-Value : 

Z-Value is the number of degrees of temperature change necessary to change the D-Value by a Factor of 10.
For example, if D-Value at 121ºC is 1.5 min & Z-Value is 10ºC.
Then D-Value at 131ºC will be 0.15 minute.

Related: Determination of Z-Value for Biological Indicators

Sterility Assurance Level (SAL) : 

SAL of a Sterilization process is the degree of assurance with which the process in question renders a population of items sterile.
Required SAL for steam sterilization process    = 10-6 or better


Characteristics of Biological indicator (IP 2007)
Sterilization Mode
Examples of Typical D-Value
(minutes)
Minimum D-Values
for selecting a suitable biological indicator (minutes)



Minimum Survival Time
(minutes)

Kill Time
(minutes)
Moist Heat 121º
1.9
Min 1.5
Max 3.0


Min 4.5
Max 14.0
Min 13.5
Max 32.0

Formulas Related to Autoclave Validation

Fo = D121 ( logNo – log N )
Where No= Initial spore population (B. stearothermophilus) N = Spore population after sterilization cycle.
For example, if D121 = 1.5 min, No=106, N=1(for SAL of 10-6)
Fo = 1.5 (log106 – log 1) = 9 min
So to achieve 6 log reduction (6 D) 9 minute sterilization cycle will have to be run.
Fo = Δt Σ 10(T-121)/z
Where Δt = Time interval between two following temperature measurements (1 minutes), T = Observed Temperature at that particular time, z = Z-Value

Also see: Log reduction and SAL value
Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
Email: .moc.enilediugamrahp@ofni Need Help: Ask Question


2 Comments so far : Add yours...

Aum Advance Research Laboratories said...

if at 121C sterilization for 15min growth of microorganism comes in my media and i want to run the cycle at set point of 122.5C for 15-16min, is it possible to justify this cycle?. how can i calculate the F0 value for the same and what is the miniumum and maximum range of F0 value.

Ankur Choudhary said...

You should increase time for sterilization instead of temperature.

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