What you are referring to relates to the requirement of ISO 17025 clause 7.7 to ensure the validity of results. Internal quality control can include, for example, the laboratory or quality manager submitting control samples as unknown test samples. This is listed as clause 7.7.1 g, retesting of retained items. Simply ensure the samples are processed as routine samples and monitor the results against expected results. When it comes to exchanging samples externally for quality control purposes, ensure best practices are followed and the approach to statistical evaluation and performance criteria are agreed to in advance. This is listed as clause 7.7.2b, participation in interlaboratory comparisons other than proficiency testing. In both cases, clause 7.7.3 states the requirement to analyze data and if the results are not within the pre-defined criteria, the laboratory must take corrective action.