In microbiology, D-value refers to decimal reduction time (or decimal reduction dose) and is the time (or dose) required at a given condition (e.g. temperature), or set of conditions, to kill 90% (or 1 log) of the exposed microorganisms. The term originated from assessing microbial thermal resistance and thermal death time analysis; however, it has analogous uses in other microbial resistance and death rate applications, such as (but not limited to) those of ethylene oxide and radiation processing.
Thus after a colony is reduced by 1 D, only 10% of the original organisms remain, i.e., the population number has been reduced by one decimal place in the counting scheme. Generally, each lot of a sterilization-resistant organism is given a unique D-value. When referring to D-values, for the purpose of thermal analysis, it is proper to give the temperature as a subscript of the "D". For example, given a hypothetical organism which is reduced by 90% after exposure to temperatures of 150° C for 20 minutes, the D-value would be written as D150C = 20 minutes. If generally describing D-value for any temperature, a common abbreviation is DT (where T is temperature) until a value for T is relevant to express, specifically. Another more general abbreviated expression of D-value is D10 (as to denote 10% reduction).
D-value determination is often carried out to measure a disinfectant's efficiency to reduce the number of microbes present in a given environment.