Because there is no one single test that can diagnose MS, doctors rely on evidence gathered from clinical examination, patient history, and the results from various tests. But as the last two decades have brought new technology and a new understanding of the disease, the criteria that doctors use to make a diagnosis have also changed.

In April 2001, an international panel in association with the NMSS recommended revised diagnostic criteria for MS. These new criteria have become known as the McDonald criteria after their lead author, Dr. W. I. McDonald. They make use of advances in MRI imaging techniques and were intended to replace the twenty-year-old Poser criteria.

It is helpful to understand the criteria that doctors use to make an MS diagnosis as it explains what your doctor is looking for and why — in some cases — a definite diagnosis is delayed. In the past, the basic criteria for an MS diagnosis were:

• There must be objective evidence of two relapses, where new symptoms appear or prior symptoms become worse for at least twenty-four hours.

• The two relapses must be separated in time by at least one month. And they must show evidence of having occurred in different areas of the CNS.

• There must be no other explanation for the symptoms.

With the recently developed McDonald criteria, doctors can utilize new diagnostic tools, especially MRIs, to make an MS diagnosis. The need for a speedy and accurate diagnosis is especially important, since disease-modifying drugs are available.