Hello MSdx,
When I was going through the initial diagnosis process the final test was a spinal tap. The MRI's of the brain were normal at that time. They had ruled out ALS, rheumatoid arthritis, lupus, HIV and Lyme disease. After doing c-spine MRIs, with contrast they found lesions. And the spinal fluid had the protein that made them decide it was MS.
It is not something anyone wants to have done, but unfortunately it does end up being the thing that confirms a diagnosis.
Best of luck!
Kim