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D - Value, Z - Value and F0 - Value Calculations


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

F0-Value :

F0-Value at a perticular 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)
Fo-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)

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




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