An MD&DI January 1997 Column
Device manufacturers face a number of challenges when trying to bring
their products to market. In addition to designing and manufacturing devices,
they must ensure that the devices are adequately sterilized. The sterilization
process requires manufacturers to perform such tasks as validation and bioburden
Marjorie Lang, a consultant with Lang Consulting, Inc. (Minnetonka, MN),
instructs readers to revalidate their sterilization processes when changing the
levels of sterilants. She also explains why bioburden studies are necessary for
devices to be sterilized with gamma irradiation.
Q. Our firm plans to change its sterilization procedure to use a
10/90 ethylene oxide (EtO)/hydrochlorofluorocarbon (HCFC) cycle instead of a
12/88 EtO/Freon cycle. Do we need to revalidate our process, or can we proceed,
using this drop-in gas replacement?
A. Companies that make such a change need to revalidate their sterilization
process for microbial lethality, package integrity, EtO residual levels, and,
depending on the design of some devices, poststerilization performance
characteristics. The reason for this is that, even though HCFC carrier gases
were designed to duplicate the chemical inertness and blending characteristics
of Freon, EtO/HCFC mixtures contain slightly less available EtO and possess
slightly different physical properties in regard to vapor pressures and boiling
points. Such differences necessitate the use of lower evacuation levels and
higher process pressures than were typically needed in 12/88 cycles in order to
obtain the desired EtO concentration. In some cases, a slightly longer gas
exposure time may also be needed. Additionally, unless the sterilization chamber
has been refitted, chamber fill rates may also be affected because of the slight
difference in the amount of heat required for chamber charging.
Q. Why do we have to evaluate product bioburden for devices we intend to
sterilize with gamma irradiation? Can't we simply process the devices at a high
dose level and assume that the process will result in at least the minimum
sterility assurance level?
A. There are two important reasons for matching the level of gamma irradiation
to the product bioburden. First, researchers need to determine the amount of
product bioburden and demonstrate that it is consistent, so that a baseline
number is available to calculate the sterility assurance level conferred by the
Second, gamma irradiation will always cause some change to materials, either
immediately or over time, and these changes are best minimized by using as low a
dose as possibleeven if the materials initially appear capable of withstanding
a high dose. Because odd radioresistant organisms do exist, and bioburden levels
can spike or drift out of control, sterilizers need to audit doses quarterly to
ensure that the minimum sterility assurance level is being achieved.
"Help Desk" solicits questions about the design, manufacture, regulation, and sale of medical products and refers them to appropriate experts in the field. A list of topics previously covered can be found in our Help Desk Archives. Send questions to Help Desk, MD&DI, 11444 W. Olympic Blvd., Ste. 900, Los Angeles, CA 90064, fax 310/445-4299, e-mail [email protected]. You can also use our on-line query form.
Although every effort is made to ensure the accuracy of this column, neither the experts nor the editors can guarantee the accuracy of the solutions offered. They also cannot ensure that the proposed answers will work in every situation.
Readers are also encouraged to send comments on the published questions and answers.