The 2024 McDonald Criteria Explained: What Patients Need to Know About the New MS Diagnosis Guidelines

The rules for diagnosing multiple sclerosis have been updated. The revision is known as the 2024 McDonald criteria, after the year the international panel agreed on it, although the full paper was published in The Lancet Neurology in September 2025. It is the first major update since 2017, and it changes how MRI scans, spinal fluid tests, and eye tests are combined to say yes, this is MS, or no, it is not.

If you are reading this because of new symptoms, an MRI report that mentions lesions, or a diagnosis you are not sure about, this guide explains what changed and what it means for you, in plain language.

Are you worried you might have MS?

Waiting for answers is often the hardest part of an MS evaluation. A few things are worth knowing before you go further.

• Not everyone with white matter lesions on MRI has MS. Migraine, small vessel disease, and ordinary aging produce similar-looking spots.
• Meeting some of the criteria is not a diagnosis. MS is diagnosed only after conditions that mimic it have been considered and there is no better explanation for the findings.
• The criteria are a tool for specialists, not a checklist to apply to yourself. They require an examination, a careful reading of the actual MRI images, and often spinal fluid results.

Knowing what your neurologist is looking for will help you ask better questions. It will not, and should not, replace the evaluation.

What the McDonald criteria are

The McDonald criteria are a set of rules that combine three kinds of evidence: your symptoms and examination, your MRI scans of the brain, spinal cord, and now optic nerve, and laboratory tests, usually on spinal fluid. They were first published in 2001 and revised in 2005, 2010, 2017, and now 2024.

Two ideas sit at the heart of every version. Dissemination in space means MS-typical damage in more than one part of the central nervous system. Dissemination in time means evidence that damage has happened on more than one occasion. Earlier versions often required waiting for a second attack or a new lesion to prove the second point. The 2024 criteria allow laboratory and imaging evidence to stand in for that waiting in many situations.

Why does the 2024 update matter?

The update was driven by two problems at once. Some people were waiting months or years for a diagnosis that was already clear from their scans, and some people were being diagnosed with MS who did not have it. The new criteria are designed to:

• Diagnose true MS earlier, sometimes at the first visit if the evidence is strong enough
• Use more specific evidence, including MRI features that are characteristic of MS and a newer spinal fluid marker
• Apply one rulebook to relapsing MS, progressive MS, children, and older adults
• Reduce misdiagnosis in people whose MRI spots are more likely to come from blood vessel disease or migraine

Earlier diagnosis matters because treatment works best before damage accumulates. Avoiding misdiagnosis matters because MS treatments are powerful medicines that no one should take for a condition they do not have.

What makes MS more or less likely

Before the criteria are applied, a neurologist forms a judgment about how likely MS is in the first place. That judgment shapes how much weight each finding carries, and it is where specialist experience matters most.

Findings that make MS more likely include an age between roughly 20 and 50 at onset; a typical attack such as optic neuritis, a band of numbness, or one-sided weakness that built over days; lesions in several of the characteristic locations, especially in both brain and spinal cord; oligoclonal bands or an elevated kappa free light chain index in spinal fluid; and MS-specific MRI features such as the central vein sign.

Findings that make MS less likely include a first presentation after 50 with no earlier neurological episodes; high blood pressure, diabetes, smoking, or high cholesterol; a long history of migraine; lesions confined to one region or in locations that do not fit MS; and a symptom picture that another condition explains better.

None of these settles the question on its own. A 55-year-old with vascular risk factors can still have MS, and a 30-year-old with a typical MRI can still turn out to have NMOSD or MOGAD. What the 2024 criteria do is spell out how much extra evidence is needed when the picture is less typical.

Key changes and why they matter

1. MRI is central to the diagnosis

The 2024 criteria make clear that imaging is essential in nearly every case of suspected MS. Your neurologist will use a brain MRI, a spinal cord MRI, and sometimes an MRI of the optic nerves to look for typical lesions and to rule out other explanations. A purely clinical diagnosis without imaging is no longer recommended.

For you, this means your scans carry a great deal of weight, and the way they are read matters. Bring the actual images, not just the written report, to any MS evaluation. Our guide to understanding your MS brain MRI explains what the radiologist and neurologist are looking at.

2. The optic nerve now counts as a fifth location

MS lesions are counted in characteristic regions of the central nervous system. There used to be four: periventricular (around the fluid-filled spaces deep in the brain), cortical or juxtacortical (at or near the brain's surface), infratentorial (brainstem and cerebellum), and spinal cord. The 2024 criteria add the optic nerve as a fifth.

Optic nerve involvement can be shown by MRI of the orbits, by optical coherence tomography (OCT, a quick and painless retinal scan), or by visual evoked potentials (VEPs, which time how fast visual signals reach the brain). If you have had optic neuritis, with blurred or dim vision in one eye and pain on eye movement, that episode can now officially count toward dissemination in space. This is one of the changes most likely to speed up diagnosis, because vision problems are a common first symptom.

3. A second spinal fluid marker: the kappa free light chain index

Spinal fluid obtained by lumbar puncture has long been tested for oligoclonal bands, a sign of immune activity inside the central nervous system. The 2024 criteria accept the kappa free light chain (kFLC) index as an equivalent marker. The kFLC index is an automated, numerical test that many laboratories can run more quickly and with less interpretation than the traditional band pattern.

Either marker can now substitute for dissemination in time. In practice, that means a positive spinal fluid result can allow a diagnosis without waiting for a second attack. Blood biomarkers such as neurofilament light chain are not part of the diagnostic criteria, but they are increasingly used to monitor MS once it is diagnosed.

4. Two MS-specific MRI signs: the central vein sign and paramagnetic rim lesions

Many MS lesions form around a small vein. On specialized MRI sequences, a tiny vein running through the middle of a lesion is called the central vein sign (CVS). The criteria treat a scan as CVS-positive when six or more lesions show a central vein, or a majority of lesions when fewer than ten can be assessed. Paramagnetic rim lesions (PRLs) are chronic lesions with a dark rim on iron-sensitive sequences, reflecting slow, smoldering inflammation at the edge. Even a single PRL is considered highly specific for MS.

Neither feature is required for a diagnosis. They are used when the picture is ambiguous: when lesions could be from migraine, aging, or small vessel disease, when only one region is involved, or in incidental findings. They require MRI sequences that not every center acquires routinely, so ask whether your scan included them.

5. MS can be diagnosed before a first attack in some people (RIS)

Sometimes an MRI done for headaches or after a head injury shows lesions that look like MS in a person who has never had an MS attack. This is called radiologically isolated syndrome (RIS). Under the 2024 criteria, MS can be diagnosed in this situation when there are typical lesions in at least two of the five locations and at least one of the following: new or enhancing lesions over time, positive spinal fluid, or a CVS-positive scan.

This is a significant shift. It means some people may be offered treatment before symptoms ever appear. Whether that is the right choice depends on the individual picture, and it is a decision to make carefully with an MS specialist rather than something that follows automatically from a scan.

6. Extra safeguards for people over 50 and those with vascular risk factors

White matter spots become more common with age and with high blood pressure, diabetes, high cholesterol, smoking, and migraine. In these groups, brain lesions alone are not reliable evidence of MS. The 2024 criteria ask for at least one of the following in addition to brain lesions before MS is diagnosed: a typical spinal cord lesion, positive spinal fluid, a CVS-positive scan, or a paramagnetic rim lesion.

If you are in one of these groups, expect your neurologist to be deliberately cautious. That caution protects you from being treated for MS when the real explanation is vascular or migraine-related.

7. Less waiting for proof over time

Under earlier criteria, many people were told their findings were suggestive but that a second event was needed to confirm. The 2024 criteria relax this in two ways. If you have a typical attack, lesions in at least two of the five locations, and positive spinal fluid, MS can be diagnosed without waiting. And if typical lesions are present in four or five of the five locations, MS can be diagnosed without either spinal fluid or a second event.

For many people this removes months of uncertainty, and it means treatment can begin before a second attack causes damage that might have been prevented.

8. One framework for relapsing, progressive, and pediatric MS

Relapsing and progressive MS now share the same MRI and spinal fluid rules. A progressive course still has to be documented as gradual worsening over at least twelve months, with particular attention to spinal cord lesions. Children and teenagers are diagnosed with the same core criteria as adults, with one addition. The criteria require a cell-based MOG antibody test in every child under twelve before MS is diagnosed, and recommend it at twelve and over when the presentation has atypical features, because MOGAD is a common mimic at that age.

How doctors use the 2024 McDonald criteria in practice

At an MS evaluation, the criteria are applied in a fairly consistent order.

1. History and examination. Your neurologist listens for a typical attack or a progressive course and examines you for objective signs such as reflex changes, an eye movement abnormality, or a sensory level, which point to specific parts of the nervous system.
2. Imaging. Brain and spinal cord MRI, with optic nerve imaging or OCT when vision has been involved, are reviewed for lesions in the five locations and for activity over time.
3. Spinal fluid, when needed, is tested for oligoclonal bands or the kFLC index. Not everyone needs a lumbar puncture. It adds the most when the MRI is suggestive but not conclusive.
4. Ruling out mimics. Blood tests and sometimes additional imaging exclude infections, vitamin deficiencies, NMOSD and MOGAD, vascular disease, and other explanations.
5. Applying the criteria to your specific picture: relapsing MS, progressive MS, or MS diagnosed from an incidental finding.
6. Planning treatment and follow-up once the diagnosis is confirmed, including how a disease-modifying therapy is chosen.

Visual guide

Several kinds of evidence, one diagnosis

• MRI patterns: The location and character of lesions contribute to the diagnostic picture.
• Optic nerve evidence: The optic nerve is included as an additional location in the revised criteria.
• Spinal fluid: Immune markers can add supporting evidence when testing is appropriate.

What do these changes mean for you?

If you are being evaluated now, you are less likely to be left in limbo. Strong MRI and spinal fluid evidence can be enough for a diagnosis without waiting for a second attack, and MS-specific MRI features can settle borderline cases.

If you are over 50 or have vascular risk factors, expect a higher bar of evidence before anyone calls your MRI "MS". That is a feature of the new criteria, not a sign that your concerns are being dismissed.

If you were told years ago that your findings were "not enough for MS", those scans and results may now meet criteria. A fresh review under the 2024 framework is reasonable, particularly if you have had symptoms since.

If your diagnosis is still uncertain after a full evaluation, that is not a failure of the process. You may hear the older term clinically isolated syndrome, or "possible MS", or RIS. The 2024 criteria leave fewer people in that category, but not none. What you should come away with is a specific monitoring plan: when the next scan is due and which changes would confirm the diagnosis or prompt treatment.

Next steps if you're concerned about MS

• See a neurologist who applies these criteria regularly. A multiple sclerosis evaluation reviews your history, examination, and actual MRI images together, and arranges spinal fluid testing or optic nerve imaging where they would change the answer.
• Gather your scans. Older MRIs are valuable for showing change over time, and the images themselves show more than the reports.
• Ask about the specifics. Useful questions include: Do my MRI findings meet dissemination in space, and in which locations? Were sequences for the central vein sign included? Would a spinal fluid test change the diagnosis in my case? Have NMOSD, MOGAD, migraine, and vascular disease been excluded? If I do not meet criteria yet, what is the monitoring plan?
• Consider a second opinion if the answer you have been given does not fit your symptoms, your scans, or your spinal fluid results. MS is a lifelong diagnosis and deserves a confident one.

If you are in California, you can request a visit in Beverly Hills or Los Angeles, or by video anywhere in the state.

Visual guide

Prepare the evidence for your visit

• Describe the timeline: Bring the onset, duration, and pattern of your symptoms.
• Bring prior imaging: Earlier scans can help the neurologist understand changes over time.
• Discuss uncertainty: Ask how the findings fit together and what still needs clarification.

References

1. Montalban X, et al. Diagnosis of multiple sclerosis: 2024 revisions of the McDonald criteria. The Lancet Neurology, 2025.
2. Cleveland Clinic Mellen Center. Diagnosing MS using the 2024 McDonald criteria.
3. Evolving the diagnosis of multiple sclerosis: a new landscape in light of the 2024 McDonald criteria. Biomedicines, 2025.
4. National Institute of Neurological Disorders and Stroke. Multiple sclerosis.
5. Specificity of paramagnetic rim lesions for multiple sclerosis. Neurology, 2025.