Interesting that you've raised the question, because I've been thinking about
it these past several weeks. Why not just stick to the same lab, which should be using the same consistent standards for testing - instead of moving around?
My primary care physician - who was doing my PSA tests for the past 8/9 years until the point he was (correctly) suspicious that I had prostate cancer - used several different laboratories over those years to process the tests. These labs were and are less expensive for the physician to use than if he'd used the laboratory of the hospital with which he's affiliated and at which I had my prostate surgery in September of this year.
With his suspicion raised, my physician conducted a second PSA test - and had it processed by the hospital laboratory, for comparison purposes.
Now that I've had my post-op PSA test (0.0), a test which was conducted at the hospital, I will continue to have the PSA test processed by the hospital laboratory in years to come . . . so that I have some better degree of assurance and comfort that the same standards are applied and the results will be more accurate.
I may be more fortunate than many other guys, though - that I live in the same city as the hospital and it's easy for me to get to/from the hospital for testing.
All of this - good 'food for thought' as we go forward. Thanks!