Definition of validity of method

Which is the definition of validity of a method?

ISO 17025 refers to ensuring the validity of results (clause 7.7), which is different to the validation of methods (clause 7.2.2). Both activities are essential to ensure a laboratory provides a valid result for a sample to a customer. 

Method validation refers to the process of experimentation and evaluation to determine the performance characteristics of the method. A method is validated when a laboratory has verified, through objective evidence and evaluation of these experiments, that the method is adequate for its intended use (fit for purpose).

Two of these parameters are precision and accuracy. An outcome of the validation is a decision regarding controls that need to be put in place to ensure the method remains valid. For example, room temperature, use of replicates samples.  This means that when running a batch of samples using that method under the same controlled conditions (as during validation experiments) a laboratory can provide a result for a sample, with some confidence that the performance (for example method accuracy and precision) of the method is still valid. Now to be assured of this when a validated method is used in a laboratory routinely, checks (controls) are performed using suitable tools and samples to ensure the validity of the method. This includes internal quality control checks were the precision for that batch is compared to long term precision through a standard deviation measurement. Bias can be measured internally and must be evaluated through participation in participating in proficiency testing or inter-laboratory comparison.

Is validity synonym of accuracy?

No, validity is not a synonym of accuracy. In summary validity of a result means that results can be trusted, and customers can make meaningful decisions using the results as the results. This is because the testing was performed in quality managed system; using a validated method (method had been verified to achieve the performance required to suit the purpose of the test - including accuracy) and the method was monitored and evaluated through internal and external checks (including accuracy).

The following coudl assist further.Have a look at the ISO 17025 toolkit at https://advisera.com/17025academy/iso-17025-documentation-toolkit/ It includes the procedure for validation and verification of methods, named Test and Calibration Method Procedure, along a Test Method Development, Verification and Validation Register and Test Method Development, Verification and Validation Record. The techniques for method validation are listed as well as the required records. 

The Quality Assurance Procedure tempalte is available from https://advisera.com/17025academy/documentation/quality-assurance-procedure/

Another relevant question and reply may be of interest is Methods verification at https://community.advisera.com/topic/methods-verification/