D-value / z-value
What a definition is not
A definition is SPEQ’s plain-language decode of how a term is used in practice, cited to the documents that define it. It is a practitioner reference, not legal or regulatory advice, it does not replace the definition in the source, and where a regulator’s wording differs the regulator’s wording governs.
The D-value is the time (at a given temperature) or dose required to reduce a microbial population by one log (90%). The z-value is the temperature change required to change the D-value by one log — it describes how sensitive an organism’s resistance is to temperature, and is the constant that lets lethality be normalized across temperatures.
D and z are the kinetic parameters behind every thermal sterilization calculation. For the reference organism in moist-heat sterilization, the standard z-value of 10 °C underlies the F0 concept; a different organism or a dry-heat process uses different values.
These parameters also define depyrogenation and radiation sterilization in analogous terms (dose to achieve a log reduction). Knowing the D-value of the most resistant relevant bioburden or the biological indicator lets you calculate how much treatment achieves the target sterility assurance level.
- —D-value = time/dose for one-log (90%) reduction at set conditions.
- —z-value = temperature change to shift D-value by one log.
- —Standard moist-heat reference z-value is 10 °C.
- —Basis for F0, SAL, and depyrogenation calculations.
Pharmacopeial sterilization chapters (USP <1229>, Ph. Eur. 5.1.1); PDA Technical Report 1.
Frequently asked questions
What is D-value / z-value?
The D-value is the time (at a given temperature) or dose required to reduce a microbial population by one log (90%). The z-value is the temperature change required to change the D-value by one log — it describes how sensitive an organism’s resistance is to temperature, and is the constant that lets lethality be normalized across temperatures.
Which regulations cover D-value / z-value?
Pharmacopeial sterilization chapters (USP <1229>, Ph. Eur. 5.1.1); PDA Technical Report 1.