The inclusion of proper positive controls in the development and validation of a container closure integrity (CCI) test method is crucial to a successful CCI program. In a previous blog, we highlighted some of these considerations with respect to the quantity and distribution of positive controls. However, there are many different ways that organizations go about creating, certifying, and using positive control packages. This blog seeks to further describe the different types of positive controls that may be included in a USP <1207> container closure integrity strategy.
USP <1207> defines positive controls as “a package with a known, intentional defect”. Positive controls, like negative controls (packages with no known or intentional defect), should “represent packages assembled in a typical manner as the product being tested using normally processed components”. There are several different types of positive controls to consider when designing a CCI study or validating a CCI test method, and not all positive controls are equally applicable to all situations. Different characteristics of positive controls, and considerations for positive control selection, are outlined below.
Creating Defects: Directly into Package Materials vs Use of Foreign Materials
Defects in positive control packages can be created by directly damaging the material of a package system component, as is the case with a laser-drilled, micron-sized holes drilled through the package material. In other cases, foreign materials are introduced to indirectly create or otherwise simulate a defect, as is the case with micro-capillary tubes of certain diameters glued into a package sidewall. For defects being created directly into the package components or materials, USP <1207> states “the defect materials of construction are identical to the package itself, thus potential test method interference due to product exposure to the package material can be readily identified.” However, when using foreign material as positive controls, “it is important to note that defects made by using an object foreign to the package (e.g., needle, film, wire plate) may display gas, liquid, or microbial leakage dynamics markedly different from that of actual defects.”
Characterizing Positive Control Defects: Quantitative vs Qualitative
Defects exhibited by positive controls may also be characterized quantitatively or qualitatively. Importantly, characterization of positive controls as quantitative or qualitative does not depend on whether the defect is created directly or indirectly in the package materials, as described above, but rather depends on what can be confidently stated regarding the leakage characteristics of the defect that the positive control contains. Defects may be considered quantitative when they are measured or correlated to a leak size or flow rate. For example, laser-drilled positive controls are certified to an optical size or correlated to a sharp-edge orifice based on air-effective flow rates. Similarly, microcapillary tubes typically have tight internal diameter tolerances, and path length can be considered to correlate the effective flow rate with observed leakage.
In contrast, qualitative defects are those that are best ‘described’ but are not or cannot be easily associated with a leak size or flow rate. For example, placing a 120µm diameter wire between the stopper and land seal of a crimped vial may create a defect pathway, but it would be inaccurate to describe it as “a 120µm channel”, as the actual leakage will vary depending on how well the elastomer seals around that wire. While the leakage past this wire may be quantitatively measured after creation, as may be done with laser-drilled positive controls, for example, they are more appropriately described as “a channel resultant from a 120µm wire between the stopper and vial”. For this reason, qualitative defects are typically presented as ‘type’ defects. Type defects are described in USP <1207> as a “representation of realistic package flaws”, and may include things like grossly deformed stoppers, visually apparent cracks, etc. Best practices include a combination of quantitative defects and qualitative defects added to further illustrate a CCI method’s capabilities to detect defects.
Examples of Quantitatively and Qualitatively Characterized Positive Controls
Different types of quantitative defects include laser-drilled holes, capillary tubes, and certain applications of known-diameter wires or filaments. Qualitative type defects vary based on the package system they are being created on. Some packages with type defects described in USP <1207> include vials, syringes, and heat-sealed bags. However, the concept of type defects can be applied to any product-package system being tested for CCI.
One caveat on positive controls, the presence of an intentional defect doesn’t guarantee detection by a test method. For a defect to be detected, there typically needs to be a ‘flow’ of something through the defect such as gas, liquid, or pressure (based on the test technology being used). Some examples of this includes defects which may be certified to a nominal diameter with a gas, may not necessarily leak liquid, such may be the case with defects containing tortuous path lengths. Additionally, a ‘defect’ is not synonymous with a ‘leak’. Type defects which may contain an obvious visible defect may not be leaking, as may be the case with an over-crimped vial, a chip in the side of a container wall, or an elastomeric stopper with a slit or puncture that reseals. These defects would not pass a visual inspection based on their propensity to become a leak. However, if no ‘flow’ of the substance being measured under test, as described earlier, then a CCI test method may not reject the sample on physical deformity alone.
Another caveat on positive controls, a description of size of one type of defect is not always a direct correlation to the describing the size of another type of defect. The three types of quantitative defects mentioned earlier are typically described in terms of defect size, however those sizes have different meanings. The defect size used to describe a laser-drilled hole is defined either optically or correlated to an air-based leak rate. The size used to describe a micro-capillary tube is typically defined as the inner diameter of the tube, with potentially no regard to path length. A wire or filament may be defined by the outer diameter of the wire, again, with potentially no regard for path length or the nominal defect size that wire creates in the package.
Quantitative positive controls at the proposed limit of detection, as well as below and above the limit, are used to optimize test parameters in method development, and challenge the method in validation. They act as the most feasible “true standards” for leakage against which method performance is evaluated. Positive controls are used for the determination of a method’s limit of detection and range during validation. Careful consideration to the above caveats should be given when using positive controls to describe a method’s limit of detection.
The CS Analytical Team are experts when it comes to understanding and navigating positive control selection for development and validation of Container Closure Integrity test methods. We can help define the optimal sample set to set up your CCI strategy for success.