**D value**

- Possibly the most significant characteristic of biological indicators is the level of resistance. This is well-defined by the
**decimal reduction value**orin Microbiology.*D*value - 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 *D*_{121°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*

*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.

## Read More:

- Sterilization in Pharmaceuticals Autoclaving
- Difference between a Class B and Class N Autoclaves
- Bowie Dick Test
- Vacuum leak test in Autoclaves
- Types of Autoclaves
- LAL test
- LAL test Procedure
- TLC & HPTLC
- GxP in Pharmaceuticals
- Types of chromatography
- CIP and SIP in Pharmaceuticals
- Positive control and negative control in Microbiology
- Types of biological indicators for sterilization
- Sterility Assurance Level (SAL)