http://www.nationalmssociety.org/Brochures-On%20Diagnosis.asp

The Diagnosis of MS
The basic "rule" for diagnosing MS requires both of the following:

• Objective evidence of at least two areas of myelin loss, or demyelinating lesions, "separated in time and space." This means lesions have occurred in different places within the brain, spinal cord, or optic nerve-at different points in time.
  
• All other diseases that can cause similar neurologic symptoms have been objectively ruled out.

Until "1" and "2" have been satisfied, a physician will not be able to make a definite diagnosis of MS. Waiting in limbo is extremely trying. Receiving an incorrect diagnosis may be even worse. Depending on the clinical problems present when a person sees a physician, one or more of the tests described above might be done. Sometimes tests are done several times over a period of months to help gather needed information. A definite MS diagnosis satisfies the McDonald criteria, named for the distinguished neurologist W. Ian McDonald who sparked Society-supported efforts to make the diagnostic process for MS faster and more precise. The McDonald criteria were revised in 2005 to incorporate new data that should speed the diagnosis without compromising accuracy.