Possibly the most significant characteristic of biological indicators is the level of resistance. This is well-defined by the decimal reduction value or D value in Microbiology.
The D value is the time taken to decrease the population of a known microorganism by 1 log (or 90% of the population). Therefore, only 10% of the original microbial population is left after an organism is decreased by 1D. (i.e., the population number has been decreased by one decimal place in the counting scheme).
Referring to D values, it is usual to mention the temperature (°C) as a subscript to the D.
For example, a hypothetical organism is reduced by 90% after contact to temperatures of 121 °C for 1.5 min.
D value would be written as D121°C = 1.5 min.
D levels will change depending on the population challenge and the microorganism’s resistance. The D value is typically higher the longer the exposure duration and the more resistant the bacterium.
There is “overkill” built into many sterilization cycles once a D value has been determined. Either the cycle time (or sterilization dose) is simply doubled, or it is taken from a mathematically calculated Sterility Assurance Level (SAL).
Typically the SAL is established to give a sterilization cycle calculated to achieve a 12-log reduction of the challenge population.
For instance, many autoclaves are run on an overkill cycle, which involves raising the temperature or cycle duration to theoretically kill the challenge biological indicator more completely.
The acceptance criteria for the D value are defined by the US Pharmacopeia (USP), which states:
The requirements of the test are met if the determined D-value is within 20% of the labelled D-value for the selected sterilizing temperature and if the confidence limits of the estimate are within 10% of the determined D-value.
The USP and ISO 11138-14 both permit the use of three procedures to validate the D Value
which are:
The most probable number method by direct enumeration;
A fraction negative method (such as Spearman/Karber);
Assessing the D value accuracy by using the USP survive/kill calculated cycles.
The equipment required to determine the D value is a resistometer, regardless of which of the three approaches is employed. A BIER Vessel is another name for a resistometer (Biological Indicator-Evaluator Resistometer).
Z value
A Z value is defined as the number of (°C) degrees Celsius required to change a D value by one factor of 10.
For instance, increasing the sterilization temperature by 10 °C will result in a log reduction of the D value if the population’s Z value is 10 °C.
It takes at least three D value/temperature pairs to calculate a Z value.
Z values may be graphically approximated (using the line of greatest fit) or analytically determined.
Calculating Z values is helpful for determining F values (in conjunction with D values), particularly for illustrating the link between lethality.
When employing biological indicators, other elements must be taken into account. The biological indicator’s shelf life, strip size, and package size are some of these.