Food Safety Testing: Understanding Microbiological Method Validation, Verification, and Fitness for Purpose
Before conducting microbiological testing for food safety purposes, laboratories and food producers must ensure that they are using methods that are properly validated and verified. Although method validation and method verification sound similar, they're actually quite different. Understanding the difference between these concepts is crucial to ensure data integrity and protect accreditation.
Method Validation, Method Verification, and Fitness for Purpose
In essence, method validation confirms a method's performance characteristics. Method verification shows that a lab can perform the method correctly. Fitness for Purpose is a demonstration that the method delivers expected results in a previously unvalidated matrix.
Here's a deeper dive into these must-know terms:
What is Method Validation?
Method validation is testing of a method's ability to detect the target organisms under a particular range of conditions. Method validation is performed for a particular matrix category (i.e. the type of sample that will be tested, such as environmental sponges or dairy products) or subcategory (such as low moisture, low fat dairy foods like milk powder, high moisture low fat dairy products like cottage cheese, etc).
Companies that develop commercial tests or kits typically validate the method using protocols approved by AOAC, the ISO, or another standards body. A method is considered validated if it has been shown to successfully detect the targeted organisms, to the required specificity, sensitivity, accuracy, and/or precision, under a particular set of conditions.
What is Method Verification?
By contrast, method verification is testing to ensure a method works in a specific laboratory. Each laboratory must perform method verification to show that it can successfully complete a validated method and correctly detect and/or identify the target organisms. Laboratories must ensure they use an experimental design for method verification that meets the requirements of their accreditation body and any other guidelines or regulations (e.g. GLP) that might apply. Only then can the laboratory use that method to test commercial products for food safety purposes.
Things get complicated when it comes to different matrix types. A method that has been validated for specific matrices and test portion sizes may not provide accurate results when used in other situations. Some foods contain substances that can interfere with the detection chemistry of a test (e.g. pectin can inhibit detection by PCR). Other foods can interfere with the ability of microorganisms to grow in the test medium (e.g. high acidity can reduce the growth rate of many microbes or obscure some expected color changes). Additionally, some foods can physically impede the workings of a test. For example high-fat foods such as butter may need special sample preparation to ensure that any microorganisms present are moved into the aqueous phase for detection or quantification.
Laboratories then need to determine whether an existing test and its validation are relevant for a new purpose. Can a test validated for, say, ice cream be used to test yogurt? That's where fitness for purpose comes into play.
What is Fitness-for-Purpose?
A method that is fit-for-purpose will produce accurate data to the degree needed to make correct decisions in the intended application. If the method has been validated on a particular matrix it is fit for purpose. If not, the laboratory will evaluate whether a fit-for-purpose study is needed, and will consider several factors to determine how complex that study should be.
Fitness for Purpose: Approaches to Decision Making
The first step in determining fitness for purpose is to consider the matrix itself:
- Food matrix grouping. The hundreds of thousands of food products on the market can be grouped into categories and subcategories such as foods that have similar characteristics or are produced through similar processes. AOAC microbiological method validation guidelines consider eight food categories, further divided into 92 subcategories, and one environmental category with eight subcategories. Generally a validated method is fit-for-purpose for different foods in the same category and subcategory.
If further steps are needed to determine fitness-for-purpose, guidelines for matrix addition and extension studies are available from bodies such as AOAC, ISO and the FDA. In general, more extensive testing is needed when adding matrix types that have greater differences. However, this testing can be time-consuming and costly, so when determining how much testing is needed laboratories can consider:
- Public health risk. This is based on identifying the matrix-associated microorganisms that pose the greatest health risk to the public and prioritizing testing that is most likely to catch problems in detecting these organisms.
- Detection risk. This is based on determining ways in which the test is likely to fail, such as because of inhibitors or because of very different microbial loads in two food products.
For example, imagine that a laboratory wants to test for Listeria monocytogenes as a contaminant in cooked chicken. There is a validated, commercial test available that tests raw meat for the same pathogen. The laboratory needs to decide whether this test is fit-for-purpose for this use.
Because of the relatively high public health risk, they decide that more than basic verification testing is needed, and they follow FDA guidelines to run a matrix extension study with spiked and control samples of cooked chicken. In this easy example, they quickly demonstrate successful detection of the L. monocytogenes spike and show that the test is fit-for-purpose with cooked chicken.
How Understanding Method Validation and Method Verification Can Help Food Producers
Understanding method validation and method verification can help food manufacturers to be confident in their pathogen testing methods. Being aware of the related issues can help producers get set up with an accurate and cost-effective way to test future products for contamination. Selecting the right laboratory partner will help with these important decisions.
Eurofins' experts are available for consultation on method verification, validation, and fitness for purpose. We've been leaders in developing food safety techniques like whole-genome sequencing for probiotic products. We'll partner with you to develop and carry out a successful and reliable testing plan for your products. Contact us today to learn more.
