# D-value (microbiology)

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) required, at a given condition (e.g. temperature) or set of conditions, to achieve a log reduction, that is, to kill 90%(or 1 log) 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 D150C = 20 minutes. In the US, the temperature is usually indicated in degrees Fahrenheit; a notation like D230 should be understood to mean D230F (D110C). When describing D-value generally for any temperature, like in the heading of a table, a common abbreviation is DT (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]

3. ^ Priscila Gava Mazzola, Thereza Christina Vessoni Penna, Alzira M da S Martins (17 October 2003). "Determination of decimal reduction time (D value) of chemical agents used in hospitals for disinfection purposes". BMC Infectious Diseases. 3: 24. doi:10.1186/1471-2334-3-24. PMC 270032. PMID 14563217.`{{cite journal}}`: CS1 maint: multiple names: authors list (link)