Beginner's Guide to MS: Do I Have MS? What Does It Take To Get Diagnosed with MS?

Blood Tests—While there is no definitive blood test for MS, blood tests can rule out other conditions that cause symptoms similar to those of MS, including Lyme disease, a group of diseases known as collagen-vascular diseases, certain rare hereditary disorders, AIDS, and vitamin B12 deficiency.

CLINICALLY ISOLATED SYNDROME (CIS)

The term “clinically isolated syndrome” (CIS) has been used to describe a first neurologic episode that lasts at least 24 hours, and is caused by inflammation/demyelination in one or more sites in the central nervous system (CNS). The episode can be monofocal or multifocal:

Monofocal episode—The person experiences a single neurologic sign or symptom—for example, an attack of optic neuritis—that’s caused by a single lesion.
Multifocal episode—The person experiences more than one sign or symptom—for example, an attack of optic neuritis accompanied by weakness on one side—caused by lesions in more than one place.

What is the Risk for a Person with a CIS of Developing MS?
Individuals who experience a CIS may or may not go on to develop multiple sclerosis. The challenge for the physician is to determine the likelihood that a person experiencing this type of demyelinating event is going to experience a second demyelinating event in the future, thereby meeting the criteria for a definite diagnosis of MS.

High Risk—When the CIS is accompanied by MRI-detected brain lesions that are similar to those seen in MS, the person has a high risk of a second neurologic event, and therefore a diagnosis of clinically definite MS, within several years. 
Lower Risk—When the CIS is not accompanied by MRI-detected lesions, the person has a lower risk of developing MS over the same time period.

In 85% of young adults with multiple sclerosis (MS), onset is a subacute clinically isolated syndrome (CIS) of the optic nerves, brainstem, or spinal cord. Only 30-70% of patients with a CIS develop MS. When clinically silent brain lesions are seen on MRI, the likelihood of developing MS is high. MS can be diagnosed within 3 months of CIS presentation with certain MRI and CSF criteria. Disability from MS is less likely in patients with a CIS of optic neuritis or sensory symptoms only, few or no MRI lesions, a long period to the first relapse, and no disability after the first 5 years.

Source:
Neurology Center of Fairfax - Forms for New Patients with Multiple Sclerosis. 

National Multiple Sclerosis Society - Diagnosing Multiple Sclerosis.

Miller D, Barkhof F, et al. Clinically isolated syndromes suggestive of multiple sclerosis, part I: natural history, pathogenesis, diagnosis, and prognosis.  Lancet Neurology.  2005 May; 4(5):281-8.