How is MS diagnosed? Tests, MRI and criteria

The steps doctors use to diagnose MS

Most workups follow the same broad order. Each step answers a different question, and later steps depend on what the earlier ones found.

1. History. The doctor asks what changed, when, on which side, and how long it lasted.
2. Nerve exam. This checks reflexes, strength, senses and other nerve functions.
3. MRI of the brain and spinal cord. A dye may be given through a vein to sharpen the images.
4. Extra tests if needed. These include a spinal tap, eye scans and tests that time nerve signals.
5. Blood work. This looks for other diseases that can cause the same symptoms.
6. The criteria check. The doctor asks whether all the evidence meets the McDonald rules.
7. A plan. You get a diagnosis, a different diagnosis, or a clear plan to watch and retest.

The table below shows what each piece of evidence adds, and where it falls short. Keep it handy when you read a test report.

Sources: NINDS: multiple sclerosis; MedlinePlus: multiple sclerosis; Cleveland Clinic: multiple sclerosis.

Your history and the nerve exam come first

The order and timing of symptoms help a neurologist judge whether MS could explain them. You do not need medical words. Plain notes work well. For example, "my left foot dragged on walks for a week" or "my right eye went blurry and hurt when I looked sideways."

Say when each problem began, how long it lasted, and whether it went away. Mention any older episode, even one that seemed minor at the time. Bring up infections, new medicines and other health problems around the same time.

The nerve exam checks how well different pathways work. Differences between the two sides of the body can hint at where a problem sits. A normal exam on the day of the visit does not erase a past episode. But it is still useful evidence.

Sources: MedlinePlus: multiple sclerosis; NINDS: multiple sclerosis.

What MRI shows, and what it cannot

MRI is central to an MS diagnosis. It can show lesions, which are small areas of damage, in the brain and spinal cord. Doctors look at where the lesions are and what shape they take, not just how many there are. A contrast dye, given through a vein, can make the images clearer.

Bright spots alone are not enough. Many common problems leave similar marks. In one study of 110 people wrongly told they had MS, the true causes included migraine in 22% and fibromyalgia in 15%. A leading reason for the mistakes was leaning too hard on MRI spots in people whose symptoms did not fit MS.

The 2024 criteria added new MRI clues that make a scan more specific. One is the central vein sign, a small vein running through the middle of a lesion. Another is called a paramagnetic rim lesion. These are used when available, in certain cases, to add support.

A report may call a finding "nonspecific." That means the spot does not point to one clear cause. It does not mean the scan was normal, and it does not mean MS. Ask your doctor which findings look typical, which do not, and whether a repeat scan would change the plan.

Sources: NINDS: multiple sclerosis; MS misdiagnosis study (Neurology, 2016); 2024 McDonald criteria (Lancet Neurology, 2025).

Spinal tap, eye tests and blood work

A spinal tap, also called a lumbar puncture, takes a small sample of the fluid around the brain and spinal cord. The lab looks for high levels of certain proteins and immune cells linked to MS. The 2024 rules also allow a newer marker, kappa free light chains, to add support in some cases. A spinal tap can also check for other diseases that look like MS.

Eye tests look at the optic nerve. An eye scan called OCT, an MRI of the optic nerves, or a test that times signals from the eyes can find damage there. This matters more now, because the optic nerve counts in the 2024 criteria. Our page on optic neuritis explains how this nerve gets inflamed.

Blood tests cannot confirm MS, but they help rule out other causes. When symptoms or scans hint at a related disease, doctors may test for specific antibodies. One targets aquaporin-4, which points to neuromyelitis optica. Another targets MOG, which points to MOGAD. If signs outside the nervous system appear, such as joint pain or a rash, tests for lupus may follow. Our MS vs lupus guide explains that overlap.

Sources: MedlinePlus: CSF analysis; NINDS: multiple sclerosis; NINDS: neuromyelitis optica spectrum disorder; SRNA: MOGAD fact sheet.

The McDonald criteria in plain words

The McDonald criteria are the rules specialists use to decide when the evidence is strong enough to call it MS. They are not a self-test. They count how many parts of the nervous system show typical lesions, and they weigh the rest of the evidence. A panel of experts revised them in 2024, and the paper came out in 2025.

The authors say the new rules should speed up diagnosis while keeping it accurate. The safeguard is the same as before: a doctor must ask whether another disease explains the whole picture better. An expert group has published a separate guide to the many conditions that can mimic MS.

Sources: 2024 McDonald criteria (Lancet Neurology, 2025); Differential diagnosis of suspected MS (Lancet Neurology, 2023).

When the answer is not clear yet

Sometimes the evidence does not meet the rules yet. A clinically isolated syndrome means one MS-like episode lasting a day or longer. Not everyone goes on to have MS. If MRI or other tests show silent damage elsewhere, MS can sometimes be diagnosed after just one attack. A radiologically isolated syndrome is when a scan shows MS-type lesions but the person has never had symptoms.

Waiting should have a purpose. Ask what is still uncertain, which other causes are on the list, and what the next test or visit will look for. Find out who will read your results and which new symptoms should prompt a call. Keep copies of reports and the image files. They help if you want a second opinion or move to a new clinic.

Getting it right matters. The misdiagnosis study described earlier on this page is a reminder that an MS label deserves a careful, unhurried workup.

Urgent symptoms should never wait for a planned visit. Sudden weakness on one side, a drooping face, slurred speech or sudden loss of sight needs a 911 call. These can be signs of a stroke.

Sources: MedlinePlus: multiple sclerosis; NINDS: multiple sclerosis; MedlinePlus: stroke.

Who this page is for

This page is for people going through tests for possible MS, and for the family members sitting in those waiting rooms with them. It can also help anyone whose report says "white matter changes" and who wants to know what comes next. Clinicians may find it useful to share before a first visit. The wait for an answer is hard, and asking questions along the way is part of good care.