F0Value :
F0Value 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)
F0Value 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 ZValue 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 FoValue 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
Related: Importance and Determination of F0 Value in Sterilization
DValue (Decimal Reduction Value) :
It is time required at temperature T to reduce a specific microbial population by 90% or by a factor of 10.ZValue :
ZValue is the number of degrees of temperature change necessary to change the DValue by a Factor of 10.
For example, if DValue at 121ºC is 1.5 min & ZValue is 10ºC.
Then DValue at 131ºC will be 0.15 minute.
Related: Determination of ZValue for Biological Indicators
Related: Determination of ZValue 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 DValue
(minutes)

Minimum DValues
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=10^{6}, 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(T121)/z
Where Δt = Time interval between two following temperature measurements (1 minutes), T = Observed Temperature at that particular time, z = ZValue
Also see: Log reduction and SAL value
Also see: Log reduction and SAL value
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widelyread pharmaceutical blog since 2008. Signup for the free email updates for your daily dose of pharmaceutical tips.
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