Proficiency testing is mandatory or not for ISO 17025 accreditation?

Yes it is required to fullfil ISO 17025 terms and condition 

it is one of the options .

YES ITS MANDATORY. PROFICIENCY PARTICIPATION OF LABORATORY IN AN ACCREDITED PROFICIENCY SCHEME IS MANDATORY TO GET AN ACCREDITAION FROM AN ACCREDITATION AGENCY PART OF ILAC MRA

proficiency testing is one option of two options "interlaboratory comparison or proficiency test"

but it is highly selection to start with participating in proficiency test if not available then the other option accepted. 

It is mandatory. Comparative testing of identical samples can also be conducted with other ISO 17025 accredited laboratories.

Iso 17025 is very important for as to certified of handling skill of the laboratory test if you are not certified of the iso 17205 meaning to say you are not allowed to operate or to work any of that laboratory's test.

YES, PROFICIENCY TESTING IS MANDATORY FOR ISO 17025, FOR PERSONNEL PERFORMING TESTS THROUGH REPEATABLITY & REPRODUCIBLITY AND FURTHER THROUGH INTER AND INTRA LAB. TESTS.

Yes it is mandatory for ISO 17025 accreditation 

 

Proficiency Testing is very important in ISO5 to obtain a Z-score and to compare the level of efficiency with regards to other laboratories. 

Proficiency testing or Inter Laboratory comparison is mandatory for ISO 17025 accreditation.

Yes It is mandatory Requirement of ISO 17025 Standard