written by Maree Stuart
It’s the question that can excite you or make you groan, “Show me your method validation data”.
It’s not like we are scammers trying to pull the wool over the eyes of the nearest gullible person. After all, we’re professionals!
And the same response might apply when you’re asked about your method verification data. (People using standard methods, I’m looking at you!)
In the realm of science, method validation stands as a critical cornerstone, serving as a guarantee for precision, accuracy, and reliability of analytical methods. Its indispensability springs from its role in assuring the quality and trustworthiness of results, particularly in sectors such as pharmaceuticals, environmental monitoring, and food safety.
There are some things labs do well and some with some room for improvement and that’s what we’re covering in this article.
Where to start?
Let’s start at the beginning with the definitions. If you pull out your trusty copy of an ISO standard, like ISO/IEC 17025 or ISO 15189, you’ll see that there is a section on definitions. This is a very handy section for engaging with your assessors and their wild claims about how a particular clause is to be interpreted. (Hint: you might discover that the auditors don’t read and apply the definitions either, so that’s a place for a tactical advantage in the discussion.)
Here’s what ISO/IEC 17025 defines validation to be:
“verification, where the specified requirements are adequate for an intended use”
You’ll notice that validation is verification plus something about specified requirements being adequate for an intended use. That suggests some type of assessment of the method and the specified requirements.
To understand what is involved, we need to consider the definition of Verification. This is also defined in ISO/IEC 17025 as the “provision of objective evidence that a given item fulfils specified requirements”.
In plain language, validation is proving your method does what it says it does and is fit for purpose. It requires organised processes and involves a systematic examination and testing of method parameters to meet predefined acceptance criteria, thus substantiating that the method delivers consistent results within a certain context.
What are the “specified requirements”?
The specified requirements are the client’s statements about the attributes of the method. These attributes cover how the method performs in terms of the following.
- working range
- detection limit
- quantitation limit, and
- measurement uncertainty.
Discovering these attributes is a little more than repeating a test or calibration of a sample a couple of times and testing the odd calibration standard.
Just in case you’ve forgotten what each of these attributes is and how you might approach assessing these attributes, here’s our quick primer.
|Attribute||Definition||Ideas for assessment|
|Specificity||The method’s ability to discern between analyte and potential interferences||Analyse test samples containing various suspected interferences in the presence of the analytes of interest.|
|Linearity||The method’s ability to return results that are directly proportional to the concentration of analyte in the sample||Measure blank plus calibration standards, at 6 – 10 concentrations evenly spaced across the range of interest.|
|Accuracy||The method’s nearness to the true value||Measure RM using candidate method or measure matrix blanks or test samples unspiked and spiked with the analyte of interest over a range of concentrations.|
|Precision||The repeatability of results||Measure RM or surplus samples by the same analyst and equipment, same laboratory, short timescale AND measure RM or surplus samples by different analysts and equipment, same laboratory, extended timescale.|
|Working Range||The extent between upper and lower concentrations of analyte, within which the method exhibits a suitable degree of linearity, precision, and accuracy.||Measure blank plus calibration standards, at 6 – 10 concentrations evenly spaced across the range of interest.|
|Detection Limit||The smallest quantity of analyte that can be reliably detected||Replicate measurements of blank samples, i.e. matrices containing no detectable analyte or replicate measurements of test samples with low concentrations of analyte.|
|Quantitation Limit||The smallest quantity of analyte that can be reliably quantified|
|Robustness||The method’s resilience to variations in analytical conditions||Identify variables which could have a significant effect on method performance. Set up experiments (analysing RMs or test samples) to monitor the effect on measurement results of systematically changing the variables.|
|Measurement uncertainty||This is not the property of a method; it is the property of a result obtained using the method. In simple terms, it is the range within which the true result lies.||The data from assessing the above attributes are used to estimate MU, together with other information relating to traceability of the measurement result, for instance.|
Depending on whether the test is qualitative or quantitative in nature, you may not need to assess all attributes. For instance, an identification (qualitative) measurement gives a result as presence/ absence or pass/ fail so a numerical assessment of attributes such as accuracy and precision are not required. But the selectivity of the method does need to be assessed.
Bringing Efficiency into Method Validation
To get through this activity of method validation, labs need to be organized and think smart. You can do a study of both LOD, LOQ, Accuracy and Precision by careful selection of the materials to be tested or calibrated.
One of the other attributes of methods mentioned above is measurement uncertainty (MU). Many labs will estimate the MU for a method and apply this estimate to any results obtained using that method. It’s a good efficiency, but labs should be aware of the limitations of the method and other characteristics of samples to ensure that this is a valid approach for any given sample.
Is that it? Nooooooo
Like the proverbial journalist looking to pounce on the inexperienced politician who dances around the question without actually saying anything, you’ve latched on that the important question of the “specified requirements” hasn’t been answered yet. There’s no pulling the wool over your eagle eyes!
As discussed above, these stated requirements are the client’s statements about these attributes. Sometimes our lab clients don’t know how to express these requirements. They come to the lab because we’re the experts. Fair enough! Some of us have got fancy white coats after all.
Yes, some clients won’t be able to tell us about their requirements, other than they know the sample has to be tested in a NATA lab. But a little probing can help uncover why they need the testing done. That’s where the veil is lifted to reveal just what these specified requirements are.
Usually, it is regulatory requirements about having a method with a detection limit greater than “X”, or an accuracy of “Y”. Or it might be a product specification that helps us to understand these specified requirements. Really, what we’re being asked to do is to make an assessment of the fitness for purpose of the method.
Without any knowledge of these requirements, we could have a situation where a customer is paying for a very expensive test with very accurate and precise results when they only needed a test that costs a tenth of the price with a lower level of accuracy and precision and a larger detection limit. The expensive test might be good for the lab’s bank balance, but it’s not good for getting a return customer, or even getting the customer in the first place!
If the customer is well-informed about their requirements, then having done a method validation study will ensure that you know you’re up to the task and can speak with confidence about how the method is suitable for their purposes.
The complexities of modern science and the need to build a trusted relationship with clients make it imperative for method validation to be undertaken. Method validation plays an integral role in ensuring the quality and safety of outcomes, thereby bolstering trust and confidence in labs and the results flowing out of them.
If you can’t find your way to lifting the veil around method validation, we can help! We’re experts in helping labs with both the science of measurement and quality.
You don’t have to do this alone!