Hi lopsided,
I am not an expert, but here is what I learned in my process. Firstly, doctors will not encourage nor prescribe this test, unless there is some indication that there might be a problem, such as a high cup angle, unusual symptoms, or a prosthesis that has been shown to generate higher wear rates. It is my understanding that in England, it has recently been recommended that all patients showing risk factors be tested for ion levels. In my case, my doctor was kind enough to indulge my insistent need to know. The test is not cheap, it cost me over $300 just for that one test. I believe that any hospital or lab can do the appropriate blood draw, but it takes a specialized facility to do the test. My blood sample was sent all the way from our hospital lab in Idaho, to Quest Diagnostics Nichols Institute in California. There is also some concern about results of these technically demanding test being consistent from one lab to another.
Once again, in my case my doctor was very correct in assuring me that the test was unnecessary, but I am glad that I had it done. Since I am at the end of the wear in period, I doubt that I will ever have it done again, since the level should remain consistent, or possibly drop slightly from this point.
John