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Sterilization Validation - Overkill or Bioburden Methods

The Overkill and Bioburden Sterility Assurance Methods 

As with any sterilization method, the room temperature vaporized peracetic acid (VPA) sterilization process must prove sterility for new product development and materials. This process is called sterilization validation; and VPA uses one of two methods—the overkill method or the bioburden method.

REVOX® Sterilization Solutions offers validation services, and can assist in new product development with sterilization validation protocol development using either the overkill or bioburden methods. 

Overkill Method

The overkill method is most commonly used for gas- or vapor-based sterilization validation.

This method uses a biological indicator(s) populated with 106 (1 million) of the most resistant organism (MRO). A fractional (time), or sub-lethal cycle(s) is run to determine the D-Value of the product in a given sterilization process.

The D-Value represents the amount of time needed to kill one logarithm (log) of the MRO. The half cycle is determined by multiplying the D-Value determined in the fractional cycle by six, plus a safety factor. The calculated half cycle is then tested to verify that a six-log reduction is achieved in all locations within the load.

A full cycle is double the time/injection of a half cycle. This results in a sterility assurance level (SAL) of 10-6 or a twelve-log reduction. SAL 10-6 represents a 1:1,000,000 possibility of a non-sterile device.

Bioburden Method

The bioburden method is most commonly used in radiation sterilization validation, where the native (starting population) bioburden is used to determine the dose needed.

The bioburden method requires quarterly (annual basis) monitoring of your device’s bioburden levels. Using the bioburden method for gas- or vapor-based sterilization validation is slightly different than the method used for radiation. The native bioburden level is still measured, but used to determine the sterilization cycle, and the amount of most resistant organism (MRO) needed for testing.

For example, if your device has two logs of native bioburden, a two-log biological indicator of the MRO is used. A cycle is developed to kill two logs of the MRO in the hardest-to-reach locations within a load. This cycle is then multiplied to achieve a SAL of 10-6, or an eight-log reduction.

The bioburden method is typically used for devices that are too sensitive for the overkill method. This cycle is then multiplied to achieve a SAL of 10-6 or 8 log reduction. The bioburden method is typically used for devices that are too sensitive for the overkill method.   

 REVOX sterilization validation bioburden and overkill method comparison chart
               The VPA sterilization validation process, showing overkill and bioburden sterility assurance methods.