Calibration laboratories, test laboratories and medical laboratories regularly have to take part in proficiency testing as stated in ILAC document ILAC-P9. For a proficiency test various laboratories are sent the same sample, which could be blood, sludge or drinking water for example. Afterwards the measuring results are independently compared to a reference value or to the combined results of all participants.
For accredited laboratories the use of ISO/IEC 17043 accredited proficiency testing organisers is preferred.
You can use the search engine or consult the list of accredited bodies to find details of all proficiency testing organisers accredited by the RvA. Other organisers can be found on the EPTIS database (https://www.eptis.bam.de). Policy document BR010 describes the various fields of work relating testing.