Determining D-Value: The Importance of Standardized Measurements
Many factors affect sterilization; concentration of the sterilizing agent, temperature, pH, relative humidity, cleanliness of product, whether organic or inorganic matter, innate resistance & population of microorganisms, plus duration of exposure or time.
With so many variables at hand, there is a need to define an industry-wide accepted methodology that characterize sterilization in terms of correlation between duration that achieves sterilization under a standardized set of conditions.
Such duration may be addressed by calculating a D-value – decimal reduction time, which is the time required (usually in minutes) to kill 90% of the organisms being tested. Sterility is controlled by allowing for an SAL (sterility assurance level) usually at 10-3 or 10-6; this means a one in a thousand and one in a million chance respectively that a device would not be organism free. In order to prove this, one must show at least a 3 or 6 log reduction.
Most biological indicators (BIs) have a population of 106, or six logs, of colony forming units (CFU) that need to be completely killed in a half cycle for the sterilization validation’s success when dealing with medical devices.
In the overkill method, an assumption is made that some level of microorganisms (bioburden) is present on the device before sterilization. Therefore, a 12 spore log reduction (SLR) must be shown for safety and obtaining the needed SAL. One method of determining a D-value is the Stumbo/Murphy/Cochran that uses the formula D = U/ log No – log Nu ; where D is the D-value, U = exposure time, No = population of BIs used, Nu = ln (n/r) where n = total number of BIs used and r = number of negative BIs after exposure time. For the most accurate results, use ≥ 20 BIs in the run and choose a time point where approximately half your BIs are negative.
This cycle is known as a fractional or sub lethal cycle and will be much shorter than the half cycle, it will be an estimate for the cycle duration but results are acceptable if one has either 20% survival to 20% kill. Total kill or no kill renders this formula unusable.
Once a D-value is calculated, an SAL can be determined for the device with the understanding that spore kill is log linear. A two minute D-value means a 12 SLR in 24 minutes of exposure. This level provides safety for bioburden on devices while still covering the SAL of 10-6.
The use of D-value is widely accepted in the field of industrial microbiology, which includes the food industry, medical device and pharmaceutical industries. In the case of industrial sterilization, as product characteristics are developed during cycle development, it is imperative to understand the D-value of the product sterilization cycle, so one may know the effectiveness of such cycle when compared with other sterilization methods.