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F0 Value, D Value and Z Value Calculations

Learn how to calculate the D - Value, Z - Value and F0 - Value for Sterilization 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)
F0-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

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
Minimum D-Values
for selecting a suitable biological indicator (minutes)

Minimum Survival Time

Kill Time
Moist Heat 121º
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

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