I've only had one ~13 years ago. It was negative for the proteins that only a percentage of people with MS have (don't remember what % that is). But basically, if you don't test positive doesn't mean you don't have MS. That is incredibly frustrating I know.

I believe that the definitive diagnosis comes with lesions on brain/spinal cord and no other underlying issue that would cause that. So 7 years after a clean spinal tap I was formally diagnosed when symptoms picked up again. They didn't do the spinal tap again though. MRI showing lesions and clean blood tests were the deciding factor.