In microbiology, in the context of a sterilization procedure, the D-value or decimal reduction time (or decimal reduction dose) is the time (or dose of an antimicrobial drug) required, at a given condition (e.g. temperature) or set of conditions, to achieve a one-log reduction, that is, to kill 90% of relevant microorganisms.^[1] A D-value is denoted with the capital letter "D". Thus, after an exposure time of 1 D, only 10% of the organisms originally present in a microbial colony would remain. The term originated in assessments of microbes' thermal resistance and in thermal death time analysis; however, it now has analogous uses in other microbial resistance and death rate applications, such as for ethylene oxide and radiation processing.

Details

Use of D-values is based on the assumption that the procedure in question causes the number of living microorganisms to decay exponentially. From this perspective, D-values can be understood as roughly analogous to half lives of radioactive substances, however a half life involves a reduction of 50% rather than 90%. The half life is actually roughly 30% (log₁₀2 ≈ 30.103%) of the D-value, so if D = 10 minutes, the number of living microorganisms will be halved in about 3 minutes.

Generally, each lot of a sterilization-resistant organism will have its own specific D-value. Determining a D-value requires an experiment, but only gives the D-value under the specific conditions of that experiment. D-values are unique to the conditions of the environment that the bacteria currently exists in.^[2]  

In the context of thermal analysis it is typical practice to subscript the "D" with an indication of temperature. 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 D_150C = 20 minutes. In the US, the temperature is usually indicated in degrees Fahrenheit; a notation like D₂₃₀ should be understood to mean D_230F (D_110C). When describing D-value generally for any temperature, like in the heading of a table, a common abbreviation is D_T (where T stands for the temperature), where specific values for T may be given elsewhere. A numeric subscript may also be used to indicate some other level of reduction than 90%; for example, D₁₀ denotes the time required for a 10% reduction.

D-values are sometimes used to express a disinfectant's efficiency in reducing the number of microbes present in a given environment.^[3]