Diagnosing MS is challenging, because there's not a single test that can show that someone has MS, according to the National Multiple Sclerosis Society . In order to be diagnosed with MS , people have to meet three criteria: neurological evidence of damage to two areas of the central nervous system; proof that the damage happened at different times; and ruling out of all other diseases that are similar to MS. Blood tests, MRIs, spinal taps, and evoked potential tests (which measure electrical activity in the brain) may be required to diagnose someone . Some studies suggest that people with MS may develop the disease for five years before receiving a clinical diagnosis.