The 2024 McDonald Criteria: How Multiple Sclerosis Is Diagnosed Now

The short answer#

Multiple sclerosis is now diagnosed with the 2024 revisions of the McDonald criteria, published in September 2025 in The Lancet Neurology by an international committee. The update pushes in two directions at once. It lets clinicians confirm MS sooner, sometimes after a single episode, by counting the optic nerve as a site of disease and by admitting newer markers such as the central vein sign and kappa free light chains. In the same breath, it raises the bar for the patients who are misdiagnosed most often, so that a faster answer is not a less accurate one.

Key points#

• The optic nerve is now a recognized location for MS damage, alongside four regions used before.
• Two MRI findings (the central vein sign and paramagnetic rim lesions) and one spinal-fluid test (the kappa free light chain index) have been added to the toolkit.
• Positive spinal fluid can substitute for waiting to see a second attack, so proof of disease over time is no longer always required.
• Adults over 50, people with vascular risk factors, and those whose main problem is headache now need stronger evidence before MS is confirmed.

Why a diagnostic rulebook exists at all#

MS is inflammation and scarring in the brain, spinal cord, and optic nerves, and no single test settles the question. Since 2001 the McDonald framework has instead assembled a case from the clinical story, MRI, and spinal fluid. Two ideas anchor it. Damage must be spread across different parts of the central nervous system (called dissemination in space), and it must have happened at more than one moment (dissemination in time). Think of it as a detective needing both several crime scenes in different neighborhoods and evidence that they did not all happen on the same night.

The 2024 revision, assembled by dozens of experts across many countries under a high threshold for agreement, is the first complete rewrite since 2017. The problem it wrestles with is a genuine trade-off. Move too slowly and a treatable disease advances while the patient waits for certainty. Move too loosely and someone is handed a lifelong label, sometimes with immune-modifying medication, for a condition they do not have. A steady stream of studies over the past decade has shown that a real fraction of people carrying an MS diagnosis were mislabeled, frequently because unremarkable spots on a brain scan were read as more than they were.

A new location on the map#

The most straightforward change is anatomical. Older criteria counted four regions where MS lesions gather: around the ventricles, in or near the cortex, in the brainstem and cerebellum, and in the spinal cord. The 2024 version adds a fifth, the optic nerve.

That addition closes an odd gap. Optic neuritis, an inflamed optic nerve that causes painful loss of vision, is one of the classic first signs of MS. Yet under the previous rules that very event did not formally help show the disease had spread. Now it can, supported by the clinical history plus tests such as optical coherence tomography, visual evoked potentials, or dedicated MRI of the nerve. Demonstrating involvement of at least two of the five regions is the usual way to satisfy dissemination in space.

The markers that can replace waiting#

The central vein sign#

MS lesions tend to grow around a tiny central vein, a fingerprint that shows up on particular MRI sequences. The ordinary white-matter spots that come with age or blood-pressure problems generally lack it. The 2024 criteria let this central vein sign help confirm MS, which is most useful precisely where the two look alike, separating genuine demyelination from vascular wear. A related finding, the paramagnetic rim lesion, marks smoldering long-term inflammation and is recognized as further support.

Kappa free light chains#

For decades the spinal-fluid case for MS rested on oligoclonal bands, a hands-on laboratory test showing that the immune system is manufacturing antibodies inside the central nervous system. The 2024 criteria accept the kappa free light chain index as an interchangeable stand-in. It runs on an automated analyzer, depends less on who is interpreting it, and reported accuracy is on par with oligoclonal bands. Either positive result can now take the place of watching and waiting for a second attack.

Toward a biological definition#

Because these markers can stand in for the passage of time, dissemination in time is no longer mandatory in every case. The committee frames this as a move toward defining MS by its underlying biology, echoing how fields such as Alzheimer's and Parkinson's disease increasingly anchor a diagnosis in pathology rather than in a repeat of symptoms. In practice that can mean a diagnosis at the first clinical episode, and in carefully chosen instances of radiologically isolated syndrome, where MS-like lesions turn up by accident, even before any symptom appears.

The safeguards that come with speed#

Diagnosing earlier raises the cost of being wrong, so the revision writes its cautions into the rules instead of hoping clinicians remember them.

The clearest example targets the people in whom small white-matter spots are both common and usually harmless: adults over 50, those with vascular risk factors such as high blood pressure, diabetes, high cholesterol, or smoking, and those whose leading complaint is headache. For these groups the criteria demand stronger evidence, for instance a spinal cord lesion, positive spinal fluid, or the central vein sign, before MS is confirmed. Children get their own caution, because their alternative diagnoses differ from those in adults.

The reasoning is that any test is only as informative as the person you apply it to. In a young adult with typical optic neuritis, a supportive MRI finding carries real weight. In a 60-year-old with vascular risk and vague spots, the same finding needs backup. Requiring the more specific markers exactly where false alarms cluster is how the committee bought earlier diagnosis without inflating the error rate.

What it means for a patient today#

If you are under evaluation, the 2024 framework can shorten your limbo, and in the right circumstances deliver an answer from one well-documented episode backed by MRI and either spinal-fluid or vein-based markers. It also means your workup may not resemble a relative's from ten years ago, with newer MRI sequences and the kappa free light chain test sitting beside the familiar ones. These criteria remain specialist tools, and applying them still turns on your whole clinical picture rather than any single result.