In Analysis of the Relationships Between Proficiency Testing, Validation of Methods and Estimation of Measurement Uncertainty, by Albano and Caten, Accred Qual Assur (2016) 21:161-166, they point out the necessity of these processes to ensure quality results. Method validation is important, and is "the confirmation, through objective evidence, that the specific measurement requirements are met for an intended use." It looks for where lab errors can occur. And "therefore, it is important to validate every stage of the test or calibration in the laboratory environment." So, did they properly perform the method validation, and have they checked it with their Proficiency testing.
Forensic crime labs rely upon institutional knowledge, and work performed years ago if ever, to show they have performed method validation. They then attempt to use what they call proficiency testing to justify their results. However, if they have not properly performed, or can prove, the existence of the method validation, they cannot prove the reliability of the results. Another jewel in the forensic library.