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 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. 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.