My horse was tested via UC Davis recently, with the results coming back as negative. I took him to a large, well known clinic for a comprehensive neuro workup, which included a spinal tap. The spinal tap came back positive. Both my regular vet and the specialist at the clinic indicated that a spinal tap is the best test for determining EPM status. This was only a month ago.
1. They don't really do spinal taps for EPM anymore - they do a blood titer that is sent to UC Davis and is quite accurate as to exposure levels. Treatment is often based on the results of this test. The test is around $100, I believe, and is well worth doing.