Sterilization value: definition
The Sterilization Value F₀ is expressed in units of time and is used to quantify the effect of a sterilizing treatment. In other words, it is a thermo-biological function, expressed in minutes, that quantifies the lethal effect of moist heat on viable organisms.
The estimate corresponds to the approximation necessary for programming an automated system designed to reach a minimum final sterilization value.
This calculation method enables the sterilizing effect to be measured at a reference temperature, linking the reduction of biological contamination proportionally to the exposure time of that contamination to thermal energy.
In practical terms, it is also of interest in enabling the comparison of different moist heat treatments (saturated steam, air-steam or superheated water). For example, a comparison can be made between the efficacy of sterilization cycles relative to a regulatory target, or between cycles themselves, to determine which are the most effective, the most economical or the least damaging for a heat-sensitive load.
Calculation of the sterilization value
The sterilization value F₀ is defined as the sum of the sterilizing effects per unit of time over the entire duration of the phase declared as sterilizing in a sterilization cycle.
In practice, it is the area defined between the baseline — i.e. the threshold temperature from which the sterilizing effect is quantified (often 100°C) — and the profile of the curve plotted during treatment of the load. This can be easily calculated mathematically by integrating the temperature/time pair, and graphically using the trapezoidal method.
The sterilization value F₀ therefore enables the thermal efficacy of different treatments to be compared by referring everything to the reference conditions (Z = 10°C and T = 121.1°C).
F0 = ∫ 10((T-Tref)/Z) .Δt
The law of decline in the number of micro-organisms as a function of time at constant temperature
Since sterilization is a bimolecular chemical reaction between the microbial structure and the sterilizing agent (e.g. water vapour), experience has demonstrated the validity of the first-order kinetic law model, which describes, at constant temperature, the logarithmic decline in the number of micro-organisms as a function of time.
A true dose-effect relationship links the biological effect to the absorption of moist (or dry) heat.
Initial and final concentration
Since sterilization is an operation that progressively reduces the initial concentration of micro-organisms until a predetermined or regulatory objective is reached (the final concentration), it is first necessary to know the initial concentration or bioburden, and to define its unit.
- The initial concentration (N₀) is either measured in a statistically significant manner, or more commonly estimated and over-evaluated using the overkill method, if the product is not thermolabile.
- The final concentration (N) — obviously much lower than N₀ — is expressed as the Probability of a Non-Sterile Unit (PNSU) on or in the treated unit. This can never be greater than 10⁻⁶: i.e. a 1-in-a-million "chance" of retaining a viable micro-organism in that unit if it must comply with European regulations for sterile human injectable products.
Although statistically this PNSU linked to the unit can also be extrapolated as one possibly contaminated unit per million treated units, in practice it is necessary to additionally factor in the probability of treatment homogeneity applied to the load, reproducibility throughout the year, as well as the many variable parameters that may influence the bioburden (raw materials, washing and disinfection, air handling conditions, personnel…).
Decimal reduction time: D_T
At a given temperature, the decimal reduction time D_T corresponds to the time required to reduce the population of micro-organisms by a factor of 10, i.e. 90% or 1 log.
To define the time-temperature combination for a sterilization cycle of an injectable solution, a meaningful comparison of the D values of the local flora and at minimum of the reference strain is essential — bearing in mind that this same D value can vary by 100% to 500% depending on the strain, the dilution medium and in particular the saline concentration or the presence of Ca²⁺, Mn²⁺, Mg²⁺ and KCl ions. Depending on elastomer formulations, D also varies by a factor of 1 to 3.
Having determined the sterilization value F₀, we know the reference temperature, the required exposure time, and therefore the well-known time/temperature combination. Example recommended by the European Pharmacopoeia: 15 min at 121.1°C.
The logarithmic law governing the equivalences of a lethal biological effect at variable temperatures
The Z coefficient, expressed in degrees Celsius, characterises the temperature variation that modifies the resistance of a micro-organism (D value) by a factor of 10, i.e. 1 log. The reference Z value adopted for moist heat is that of Geobacillus stearothermophilus, for which Z = 10°C for moist heat.
In order to compare different thermal treatments, the general function F(Z) — of which the values F₀ and F_H are special cases — enables efficacies to be compared according to time/temperature combinations. This function can be extended to other treatments, but only thermal ones: reference is made to the pasteurization value P₀, cook value C₀, depyrogenation value F_T and disinfection value A₀.
When Z = 10°C and T = 121.1°C, F_Z = F₀ in the context of moist heat sterilization.
Note: the scientific community agrees in recognising a good representativeness of the F₀ model for temperatures in the range of Tref ± 2Z.
Example
How to compare the efficacy of 2 moist heat treatments: one at 121°C for 20 min and another at 118°C for 30 min
Using the Z value, it is possible to determine, for 1 minute of treatment at a temperature X, what is its equivalent sterilizing effect at the reference temperature.
For example, 1 minute at 113.6°C in moist heat guarantees, according to this model, the same sterilizing effect as 0.1774 min at 121.1°C. Equivalence calculations are available in the table above.
If, in our example, we must achieve a 15-log reduction — i.e. 15 min at 121.1°C with a D value of 1 — then the duration of the sterilizing phase at 113.6°C will be 15/0.1774, approximately 1 hour 25 minutes.
Another example: 2 min at 118.1°C or 0.5 min at 124.1°C are equivalent to 1 min at 121.1°C.
Therefore, in the example given, 30 min at 118.1°C (equivalent to 15 min at 121.1°C) is a treatment approximately 25% less effective than 20 min at 121.1°C.