Method One, commonly called the Bioburden Method, is based on an assumed number of organisms residing on the product prior
to sterilization. A database of typical industry bioburden levels is used to determine this assumed number of organisms, and
represents a broader, more stringent challenge than the natural bioburden found on a device would. Ten samples from each of
three lots are tested for a total of 30 samples. The bioburden results of these samples are used to calculate an experimental
radiation dose called the verification dose, which is anticipated to yield a SAL of 10-2. An additional 100 samples from a single production lot are exposed to this dose and sterility tested. If there are no more
than two nonsterile cultures in the 100 sterility test samples, the validation is considered successful, and a routine SAL
sterilization dose is calculated based on the original bioburden data.
A bacteriostasis or fungistasis test is also conducted with selected microorganisms to examine whether the presence or absence
of various other substances inhibits their growth. Additional samples are required for this test.
By following the above method, a minimum radiation dose that will routinely provide the appropriate SAL can be established
for a filter capsule. When the filters are irradiated at a suitable facility, a dose range will typically be used. For example,
if the minimum dose for sterilization has been established at 25 kGy, the actual range used may be 25–35 kGy.
Dosimetric release is used to verify that the dose absorbed by the product matches that of the validated specifications. During
the irradiation process, dosimeters are placed in the irradiation chamber to measure the absorbed radiation and to perform
a dose mapping inside the chamber. The product is determined to be sterile based on the physical irradiation process data
rather than sterility testing.
Validation of Product Performance After Irradiation
After the proper dosage has been established for sterilization of a capsule filter by gamma irradiation, validation tests
need to be conducted to ensure product integrity following gamma irradiation. These tests include: filter or capsule seal
testing, physical tests (burst testing, creep-rupture testing and pressure-fatigue testing), real-time shelf-life testing,
extractables testing, and biological safety testing.
The Kleenpak Nova capsule filter validation study is used as an example of a presterilized filter validation strategy. Kleenpak
Nova capsule filters incorporate either a 10, 20, or 30 in. length standard cartridge filter in a gamma-stable polypropylene
capsule. The filters used in the capsule are also used in stainless steel filter housings.
In many irradiation facilities, the maximum dose may be as high as 50 kGy. The filter capsules used in this study were exposed
to the maximum radiation dose of 50 kGy to provide the worst case conditions in terms of gamma irradiation exposure to the
Filter Capsule Seal Integrity
The purpose of filter seal integrity tests is to confirm that the seal between the filter and the outer capsule is integral.
The testing of the seal can be achieved by performing integrity and bacterial challenge tests on typical filter capsules after
they have been irradiated.
To demonstrate seal integrity, standard (10 in.) Kleenpak Nova filter capsules (incorporating 0.2 μm-rated Posidyne filter
membranes) were used for the tests (Pall Corp., part number NP6NFZP1G). Before challenge testing, six samples were each subjected
to a gamma irradiation dose of approximately 50 kGy. The six filters were subjected to a liquid bacterial challenge using
Brevundimonas diminuta (ATCC 19146) at a minimum challenge level of 107 CFU/cm2 of effective filtration area.
Each of the six filters tested passed the forward flow integrity test and gave sterile effluent when challenge tested with
an aqueous suspension of B. diminuta. These results demonstrated the integrity of the seal for the presterilized capsule filters.
Physical tests are used to ensure that filter capsules can withstand maximum pressures and maximum temperatures during operation
after they have been subjected to gamma irradiation. The Kleenpak Nova capsule filters were designed for a maximum pressure
of 43.5 psig (3 bar) at 40 °C for continuous operation for one week (168 h).
Burst pressure and creep rupture tests are used to demonstrate maximum pressure thresholds for the Kleenpak Nova capsules.
In all cases the capsules were subjected to a gamma irradiation dose of approximately 50 kGy.