Correct me if i'm wrong, but proficiency testing is testing personnel NOT lab methods
Take your SOP, shadow employees, verify they are following SOP as written, document. Voila, you're done
I second what Scampi said.
Because I'm in manufacturing of packaging material, it's a little easier for us. What we did was take good parts and bad parts and mixed them up. Then took our key employees that are involved in approving questionable parts. Individually, tested them by writing down which parts were good and which parts were bad with comments on what were bad about them.
Then our warehouse personnel were tested on using a micrometer to test the gauge of our raw material that come in and make sure they are competent in their job. So what we had to do with them was take a couple samples of raw material and have them write down at designated points throughout the materials to determine width and gauge of material, and if they would accept or reject it.
Perhaps doing something similar in your industry to where you can have the individual tested on what is acceptable versus what is not.
Hope this helps.