by Martell Winters, Senior Scientist, RM/SM(NRCM), Nelson Laboratories
Most medical device manufacturers have to choose a sterilization process for their devices. One of the two most common forms is radiation (the other is ethylene oxide).
The most common level of radiation used to sterilize medical devices is 25kGy. For planning purposes, it is also cost-effective and one of the easiest to conduct. A majority of devices can be safely sterilized with this dosage, but there are situations where you may need different levels of radiation. The international standards for radiation sterilization are AAMI/ISO 11137-1 and 11137-2.
Here are the three radiation validation methods:
- The most common—and often the most cost effective—radiation validation process is VDmax. VDmax validation tells a manufacturer if their medical device can be sterilized with the most typical radiation dose—25kGy. The validation requires a bioburden test of 10 samples from three different batches (30 samples total). The results of the bioburden test are used in a table from the radiation standard to determine the proper verification dose. This verification dose is applied to 10 additional samples from one batch followed by a test of sterility. The validation passes if there are ≤1 sample testing positive for microorganism growth. Additional testing must be performed if there are two positives. It fails if three or more of the samples test positive for growth. In the case of a failure a higher dose of radiation or changes to the device manufacturing process must be made. VDmax also has tables for other, less often used sterilization doses.
- Manufacturers may find they need to use a lower dosage of radiation, which corresponds closer to their product bioburden count. If this is the case, Method-1 validation is a good one to use. It follows the same testing scheme as VDmax, but the test of sterility is on 100 samples and the verification dose is determined using a different table from the standard. Out of the 100 samples tested for sterility the validation passes if there are ≤2 samples which are positive for growth. If three or more samples are positive, the device will need either a higher dose of radiation or changes to the manufacturing process. This method costs more because of the larger sample size.
- If a manufacturer needs to use the lowest amount of radiation possible to sterilize their device, then Method-2 can be used. (This method is infrequently used.) It does not use an established dosing table. Instead it uses multiple radiation doses on many test samples (groups of 60 samples receiving 8 or 9 different doses) to determine at which dose the product begins to provide sterile results. The sterility test results are used with various equations to determine the proper sterilization dose.
Understanding these validations helps manufacturers in the following ways:
Development Decisions—If a manufacturer understands the radiation dosage likely needed for sterilization at the beginning of development it allows them to pick materials that can handle radiation without damage or degradation. The lowest cost method can be planned for.
Radiation Dose—If the manufacturers understand the range of radiation their device can handle, it makes it easier for sterilization outsourcers to guarantee they can hit the range.
Planning—If general radiation sterilization requirements are known up front it allows manufacturers to plan for samples and sterilization costs and processes well in advance of the release of the product.