Validating Radiation Sterilization in a Global Marketplace

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI February 1999 Column

STERILIZATION

A review of current standards and their implementation and validation and how they affect product release times to the marketplace.

When choosing irradiation to sterilize medical devices, U.S. manufacturers have historically relied on FDA guidance and generally followed validation methods prepared by AAMI. Although this is still true, manufacturers must now also consider the European Medical Devices Directive (MDD). AAMI members have been working with ISO and the American National Standards Institute (ANSI) to produce voluntary, harmonized guidance for validation and testing methods. The European Union (EU) has also enacted mandatory standards for its members. Although efforts toward harmonization are still ongoing, there have been sufficient changes in the regulatory guidance to generate questions as to how to achieve simultaneous compliance with these various standards.

GUIDANCE STANDARDS

Section 820.752 of FDA's quality system regulation (QSR) requires that "all processes used to produce medical devices be validated."¹ Manufacturers may follow accepted sterilization validation guidelines or develop their own. For companies selling solely within the U.S. marketplace, compliance with ANSI/AAMI/ISO sterilization standards is sufficient.

Any medical devices sold into EU-member countries are required to meet relevant EN sterilization standards. Fortunately, there is a good deal of congruence between ANSI/AAMI/ISO and European standards for radiation sterilization (Table I).

Table I. A comparison of the various guidance documents being used for radiation sterilization process validation. (* signifies that guidance is currently unavailable.)

The current primary ISO standard regarding irradiation of medical devices is ANSI/AAMI/ISO 11137, "Sterilization of Healthcare Products—Requirements for Validation and Routine Control—Radiation Sterilization."² Some techniques developed in previous AAMI guidelines ST 31 and ST 32 were adopted into ANSI/AAMI/ISO 11137.^3,4 The validation methods in ANSI/AAMI/ISO 11137 have been referenced in European Standard EN 552, "Sterilization of Medical Devices—Validation and Routine Control of Sterilization by Irradiation."⁵

Additional approved ISO and European standards focus on test methods to support validation programs. These documents address bioburden enumeration (ISO 11737-1; EN 1174-1) and sterility testing (ISO 11737-2).^6–8 Three segments under development—EN 1174-2, EN 1174-3, and EN 1174-4—will provide specifics on sampling methods, validation of test methods, and test methodology.⁹ Technical comparison of these documents shows great uniformity in testing methods, including incubation times, media selection, and temperature conditions used during validation and routine-audit bioburden and sterility testing.

The procedure for establishing a 25-kGy minimum sterilization dose for small or infrequent production batches is not fully harmonized. Validation was previously addressed in AAMI ST 32 as method 3. In 1997, this document was superseded by an enhanced method in AAMI 13409, "AAMI/ISO Technical Information Report (TIR) 13409 Substantiation of 25 kGy."¹⁰ At present, AAMI 13409 has been approved in the United States, but it has not been adoptedinternationally.

CHOOSING A STERILITY ASSURANCE LEVEL

The first step in ensuring medical device sterility is determining the appropriate sterility assurance level (SAL), a measure of the probability that one unit in a batch will remain nonsterile after being exposed to the sterilant. Product lot sterility can only be expressed in terms of probability. For example, an SAL of 10^-3 means that one device in a thousand might be nonsterile. Selecting the SAL occurs during the dose-setting phase of radiation sterilization validation. In many cases, the intended use of the device will dictate the need for a particular SAL. The commonly accepted SAL for invasive medical devices is 10^-6.

Some European countries, however, recognize only 10^-6 SAL for a "sterile" label claim. The European Pharmacopoeia Commission concurs. Therefore, the minimum SAL may be based on the regulatory requirements of the country in which the device will be sold as much as on the device's intended use.

LOT RELEASE CRITERIA—DOSIMETRIC RELEASE

The desired goal for radiation sterilization validation is determining the minimum exposure dose that can routinely be used to meet the preselected SAL requirements and allow dosimetric release, which is the determination that a product is sterile based on physical irradiation process data rather than sterility testing. Dosimeters measure the absorbed radiation dose (physical data) delivered to the product at given locations. These data can verify that the dose absorbed by the product meets validated specifications.

Sterilization using 25 kGy did not require validation prior to implementation of the QSR. However, tests have evidenced microbiological issues that indicate that 25 kGy may not always yield the desired product SAL.

The model microorganism population used in ANSI/AAMI/ISO dose-setting validation procedures is designed so that a 25-kGy dose is expected to sterilize a device to an SAL of 10^-6 when its bioburden—the sum population of viable microorganisms on the device—is less than 1000. Many devices, however, have a bioburden greater than 1000 microorganisms, and some microorganisms have sterilant-resistant characteristics that make them harder to kill than the model population. In such cases, 25 kGy may be insufficient to achieve an SAL of 10^-6.

ANSI/AAMI/ISO VALIDATION METHODS AND SUBSTANTIATION OF 25 kGy

Methods 1 and 2 of the ANSI/AAMI/ISO 11137 guideline involve establishing a sterilizing dose using a bioburden resistance model. Method 1 is preferred because of its reasonable cost and study time. Because it employs model population data from Whitby and Gelda that is based on historical data received from manufacturers, it provides a greater challenge than the natural bioburden on a device.¹¹ With method 2, the dose is determined experimentally based upon the resistance of the device under study.

The substantiation of 25 kGy as a sterilization dose (AAMI/ISO 13409) uses bioburden to establish a minimum acceptable product release dose of 25 kGy. Although the three methods require different numbers of samples to complete the validation, they all specify quarterly dose audits to confirm the continued validity of the sterilization dose (Table II).

Table II. A comparison of the various requirements that different sterilization modes have to verify the continued validity of their doses.

The applicability of methods 1, 2, or 25 kGy to a specific product is based on a combination of factors. Table III illustrates some of the issues to be considered when choosing which method is most appropriate for a given situation. Note that this table factors in both technical and financially based considerations.

Table III. Practical considerations when selecting which sterilization method to use for a particular application. CFUs are colony-forming units.

Method 1. This method is commonly called the bioburden method because the number of organisms on the product must be determined prior to sterilization. Ten samples from each of three lots are tested for a total of 30 samples. The bioburden results are used to calculate an experimental radiation dose called the verification dose, which is anticipated to yield an SAL of 10^-2. An additional 100 samples from a single production lot are exposed to this dose and sterility tested. A bacteriostasis/fungistasis test also is 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. If there are no more than two positive (nonsterile) cultures in the 100 sterility test samples, the validation is considered successful, and a routine SAL sterilization dose is calculated based upon the original bioburden data. Method 1 generally requires 136–146 samples and is usually considered the method of choice because method 2 requires a much larger number of test samples.

When using method 1 on a large or costly device, the manufacturer may not have to use the full number of samples noted above. For example, instead of using a full complement of complete finished devices, a large device might be divided into five portions equal in anticipated bioburden makeup, both in overall number of organisms and number of types. Twenty such devices would be cut into five pieces, yielding 100 portions with a sample item portion (SIP) equal to 0.2% of the original device.

If a large device consists of several dissimilar components, each with a different level or mix of bioburden organisms, this practice will not work. There are, however, often other ways to reduce the total number of devices needed to fulfill method 1 requirements. If an SIP of less than 1.0 is chosen, it must be validated to document the bioburden equivalency by performing a sterility adequacy test with 20 nonsterilized units that yield at least 80% positives.

Method 2. This bioburden resistance method requires the manufacturer to determine the radiation resistance of the organisms actually resident on a product. In method 2, device units from each of three production lots are exposed to incremental radiation doses (e.g., groups exposed to 2, 4, and 6 kGy, etc.) and then sterility tested. The results are used to determine a verification dose expected to yield an SAL of 10^-2. A group of 100 devices is then exposed to this verification dose. If fewer than 2 of the 100 units are nonsterile, the data are used to calculate a routine SAL sterilization dose.

Method 2 is composed of two protocols. Both require a greater number of samples during validation than the other methods. For protocol 2A—validation for normal product bioburden distribution with an SIP of 1.0 or less—the minimum number of samples used is 640; 540 are used for the incremental dose series and 100 for the verification dose experiment. For protocol 2B—validation for product with consistent and low bioburden and an SIP of 1.0 (i.e., the entire device)—approximately 580 are generally tested. In each method—2A and 2B—an extra 200 samples (100 from each of the lots not used for the verification dose experiment) must be available. It they are not consumed in the study, they will be discarded if the SIP is <1.0, or they can be returned to the manufacturer for terminal sterilization if the SIP is 1.0.

A good reason to choose method 2 is its ability to validate a lower dose than method 1. Method 2 is based on a device microorganisms' average resistance to radiation, whereas method 1 is based on a theoretical model population that may or may not be similarly resistant to radiation as the organisms under study. Based on the ability of DNA ligase to repair radiation-caused DNA damage, it could conceivably take a smaller dose of radiation to destroy the less-sensitive organisms on the actual device than it would to inactivate the model population used to establish the method 1 doses. Thus, a method 2 study on such a device would allow a lower minimum routine sterilization dose.⁹ Conversely, if the ambient bioburden organisms would require more radiation than that indicated by the method 1 chart, method 2 can be an alternative method for validation. Another tactic involves identifying ways to lower bioburden levels and revalidating using method 1.

TIR 13409 SUBSTANTIATION OF 25-kGy MINIMUM STERILIZATION DOSE

Methods 1 and 2 are not practical when there is limited test sample availability. The substantiation protocol for 25-kGy radiation sterilization was designed for use with small-volume production of fewer than 1000 units per lot, for single special-order lots and lots used in clinical trials and field studies, and for release of the first lot produced by a manufacturer. Given problems of failures associated with its use, the historical AAMI method 3 was not included in ANSI/AAMI/ISO 11137 and will not be discussed here except as a counterpoint to method 13409. The industry has had only limited experience in application of the latter method. Both methods use data developed from the inactivation of the microbial population in its natural state and are based on the probability model for inactivation of microbial populations provided in ANSI/AAMI/ISO 11137.

Method 13409 targets substantiation of 25 kGy for a single batch of products or for routine production of small batches of fewer than 1000 devices. Bioburden testing and dose-verification experiments are conducted on samples from each of three production lots.

Method 3 allowed test sample sizes to be selected based on production lot sizes from 7 to 1000 units. AAMI 13409 takes into account the pitfalls associated with taking samples from small batches and requires a minimum sample size of 10 devices for each of the bioburden and sterility experiments. The rationale for this change is that the distribution of the natural bioburden on products in batches of less than 20 may vary and not be sufficiently represented if fewer than 10 units are tested, potentially leading to validation failures.

Another significant difference between method 13409 and method 3 is the acceptance criteria when testing 30 or more samples in the verification experiment. Method 3 had allowed only one positive culture; method 13409 allows two. The acceptance limit in method 13409 is more in line with the acceptance criterion established in method 1.

Method 13409 requires only three successful verifications, as long as product batches are produced more frequently than one batch every 3 months. The verification dose is recalculated and tested for each subsequent dose audit after validation completion.

For method 13049, the average adjusted bioburden of each device may not exceed 1000 colony-forming units (CFU) per device. When working with a device with a high bioburden, it is important to consider that the 1000-CFU limit applies to average device bioburden, not to each individual sample result. Bioburden spikes—individual device unit values that are significantly higher than the average bioburden of the tested sample group—can potentially lead to a validation failure. If false spikes are used in calculating the verification and sterilizing doses, prohibitively high irradiation doses might be set. It is also possible that doses could be set too low if spikes exist but not in sufficient numbers to be factored into the dose-setting calculations. Spikes within the 100 sterility test sample lot can cause a verification dose failure.

Manufacturers should investigate bioburden spikes and determine their source. Spikes can result from the following causes: lack of environmental control in manufacturing, faulty handling during or after sampling, packaging problems, or lab contamination. For additional information regarding controlling manufacturing environments, see USP Supplement 8, 1998.¹² The destruction of microorganisms can be characterized by their survival curves, and resistance data in the literature can guide a course of action during a failure investigation.¹³ A detailed review of source-specific factors known to be associated with the product—e.g., human handling or contact with air, equipment, or water—should occur. If the organisms associated with product bioburden can be identified or characterized, it is possible to determine their resistance to irradiation. High levels of resistant microorganisms may reduce the SAL to less than 10^-6.

Manufacturers that have used method 3 should review their data in light of the AAMI 13409 substantiation method requirements. Rather than invalidating the previous results, it is possible to simply adopt the AAMI 13409 audit procedure based on the testing that has been performed, although manufacturers that use fewer than 10 samples should be aware of the increased probability of failure. For example, products composed of stainless steel/titanium are manufactured under harsh conditions that reduce the microbial populations to such low levels that the population may have a resistance probability greater than that assigned to the model population. The manufacturer should increase the test sample size whenever possible to avoid validation failures. If revalidation is required, it is acceptable to continue sterilization based on the method 3 data until the revalidation process is complete.

STERILIZATION VALIDATION COMPARED WITH USP STERILITY TESTING

Although still used for some materials produced in sterile fill operations that are not subjected to terminal sterilization, or to determine sterility of products selected from field sampling, USP sterility testing is not acceptable as a lot-release criterion for irradiated devices.

Many factors support the release of devices, and there are five steps to achieving a successful validation. They are:

The product qualification phase addresses product and packaging materials evaluation, as well as sterilization dose determination. The installation phase deals with equipment testing calibration, mapping, and documentation by the sterilization facility. The manufacturer and sterilization facility should then address process qualification, which includes establishing a product loading pattern, followed by dose mapping for the identification of minimum and maximum dose within the product load. Once the data have been collected for the first three phases of validation, the documentation is certified in accordance with ISO 9001 or ISO 9002. Routine validation maintenance ensures the validity of the sterilization dose, equipment, and dosimetry systems. (For more details, refer to ANSI/AAMI/ISO 11137.) After validation is successfully completed, each product lot is released by dosimetry.

The validation process covers the product sterility assurance requirement. USP testing cannot achieve this level of assurance because it is only conducted on a subset of the total sterilant-exposed samples. While the results are accurate for the samples tested, they cannot be extrapolated to determine the sterility assurance level of the remainder of that lot. Since the test results do not demonstrate the desired SAL characteristic, product sterility testing cannot be used in lieu of validation.

Once a validation is conducted, product lot-release sterility testing is not required as long as the manufacturer periodically conducts audits to verify continued validity of the minimum sterilization dose. Barring the need for other release tests, such as pyrogen testing, the product is ready for release if the dosimeters indicate that the required minimum sterilization dose has been achieved. Although there is no rule that says that validation can't be augmented with USP sterility testing, there are issues that render this impractical. Because a USP sterility test requires a 14-day test period, its use can create a 2-week delay for a product otherwise ready for release. Also, devices used in a USP sterility test cannot be sold, and are thus a sunk cost. If the product is expensive to manufacture, product sterility testing adds significant unnecessary expense to lot release.

BIOBURDEN AND STERILITY METHODS VALIDATION

The validation methods discussed in this article require methods developments testing to ensure the reliability of bioburden and sterility test results. These tests include bioburden recovery bacteriostasis/fungistasis and SIP adequacy (discussed previously).

Questions concerning bioburden recovery validation are becoming more frequent as manufacturers and contract sterilizers realize the importance of this test in establishing a routine sterilizing dose. Although bioburden recovery validation is not a new concept, it has become a specified part of ANSI/AAMI/ISO TIR 11737-1 and EU Standard EN 1174-1. Understanding the purpose of the recovery test is critical to understanding its use.

A bioburden recovery validation generally involves three to six samples and determines the efficiency of the bioburden testing method. Based upon the materials used for manufacturing the device and the complexity of its design, there is a possibility that a specific bioburden test can only remove a fraction of the existing microorganisms from the device for bioburden counting. The recovery test makes allowance for the residual organisms remaining on the device after the assessment, yielding either a percent recovery or a recovery factor used to adjust the bioburden counts. Because the results gained from both AAMI 11137 method 1 and AAMI 13409 rely on mathematical calculations based on the device bioburden, using the correct bioburden number is critical. An underestimated bioburden results in a lower verification dose, risking validation failure and product recall.

For example, if bioburden test results yield 750 CFU per device, knowledge of the method's efficiency may be critical to the selection of the validation test method. If the 750-CFU total is a nonadjusted figure, and a subsequent recovery study of the device indicates that the bioburden test method recovers only 40% of the organisms on the device, then the assayed bioburden must be divided by 0.4, resulting in an adjusted bioburden of 1875 CFU, well over the method 13409 maximum limit of 1000.