There is a criteria for diagnosing MS: Positive MRI, symptoms that have appeared at different places on the body and over different time spans.
Thats pretty much it. Sometimes they look for protein in the spinal fluid, but even a positive result there may not get you a MS diagnosis without a positive MRI. A positive MRI is MULTIPLE lesions. These lesions must be indicative of MS. There are other disorders that can cause lesions.

Pretty much your doctor will try to rule everything else out since your MRI came up negative.
I had negative MRI's for years and then all of a sudden one day lesions showed up. Getting an MS diagnosis can be a very long journey.
Try to remain calm. Any treatment for MS is not going to cure you anyway.

Not knowing whats wrong with you is EXTREMELY frustrating and the very worst part of it all. I used to think I was nuts! Hang in there and just prod the docs to keep looking. Sometimes you just have to wait. Eventually something will rear its ugly head and you will get a diagnosis.....whatever that may be!!