External validation of analytical methods

Method validation stems from the implementation of quality assurance systems in analytical laboratories. Its initial aim was to create a relationship of trust between a service provider and a customer. Initially perceived as a constraint, it quickly became an important issue, as analysts used to work fairly independently. This attitude soon disappeared, as the laboratory had everything to gain from providing results in which the client-decision-maker could have confidence. In fact, this approach was a great success, as demand for analyses of all kinds has only increased, and still continues to do so. In just a few years, analysts have become validation experts. However, we need to distinguish between in-house validation and external validation.

Historically, different internal validation procedures have been set up on a sector-by-sector basis, as water analysis specialists felt that they did not face the same problems as those in the steel industry. This is why there are still grey areas in the definitions. As a result, many method performance parameters are open to interpretation. The best example is the limit of quantification: it is widely used, even though there are dozens of calculation methods that produce different values. Better standardization of quality criteria calculation modes is certainly the approach that will resolve these problems.

This is also the case for external, or inter-laboratory, validation. As we shall see, the principles for conducting studies and the related calculations are all covered by various international standards. They also call on dedicated statistical methods.

Finally, validating a method means making standardized statistical calculations. This remark is not accidental, since this standardization effort is in line with obvious economic and technical concerns. Indeed, the progress made in analytical methods, and the confidence that people have been able to place in their results, have meant that they have become essential decision-making tools in trade, environmental or medical diagnostics. The main aim of external validation is therefore to provide a means of ensuring good comparability of measurements, preferably at international level.