How to validate international, national and individual laboratory methods

3 How to validate international, national and individual laboratory methods 

Method validation is not, as is often assumed, carried out to ensure whether the performance characteristics of a method are fit for purpose, but rather to establish 

If carried out correctly, the process of validation enables performance characteristics, such as bias, recovery, precision and limit of detection, to be estimated with a high degree of confidence. Whether the performance characteristics determined are deemed acceptable, or not, is another matter but the 

As already stated, prior to the start of validation, it is essential that the procedures to be undertaken be clearly and unambiguously documented. Also, when the process of validation commences, it is equally essential that the procedures that are documented be strictly adhered to. Minor changes to the procedures should not be introduced, as these could affect, adversely or otherwise, the resulting performance. The only way by which changes to the performance of a method can be assessed would be to validate a method and establish its performance, then to introduce changes to the procedures and then re-validate the method to establish whether a change in the performance had occurred. 

The estimation of recovery, carried out using spiked samples with known amounts of determinands, is often the only alternative to estimating bias when certified reference materials are not available. Notwithstanding this, spiking experiments should be used with caution, since the added determinand may not be in the same form as that contained in the matrix. In addition, the time between the addition of the determinand and its extraction needs to take into account, and allow for, potential occurrence of matrix-determinand interactions. This is not always fully appreciated, and in fairness, is very difficult to accommodate. Questions like how long should a spiked addition be left before extraction and what form should the added determinand comprise are extremely difficult to address. 

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