It can often take months or even years to receive a confirmed diagnosis of multiple sclerosis (MS). The McDonald criteria for MS is a diagnostic tool that helps doctors diagnose the disease earlier. With the 2017 updated McDonald criteria, MS can be identified more quickly than previously — and the risk of misdiagnosis is reduced.

The sooner an accurate diagnosis of MS is made, the sooner treatment can start to help slow disease progression and prevent permanent damage and disability.

MS is a disease of the central nervous system (CNS). It occurs when dysfunction in the immune system damages the coating on nerves in the brain and spinal cord, causing lesions on the brain, brainstem, spinal cord, and optic nerve. MS symptoms such as weakness, fatigue, tingling, vision problems from optic neuritis, motor disability, and cognitive impairment can resemble other neurological and autoimmune conditions. This makes a differential diagnosis challenging.

MS diagnostic criteria have developed considerably in recent decades as research and technology have advanced. The McDonald diagnostic criteria were introduced in 2001. They were the first diagnostic criteria for MS to require imaging scans, which provided more specificity and additional data. Since then, several revisions of the McDonald criteria have come out, most recently in 2017.

The McDonald criteria are designed to diagnose the onset of relapsing-remitting MS (RRMS), primary progressive MS (PPMS), or secondary progressive MS (SPMS) and evaluate the development of new lesions. If you are newly diagnosed with MS or facing a potential MS diagnosis, it can be helpful to understand how a diagnosis is made and why it may take time.

How Do the McDonald Criteria Work?

The McDonald criteria are based on several factors that can help clinicians determine disease activity in the CNS specific to MS. Diagnosis should be conducted by a neurologist with expertise in MS.

The McDonald criteria are designed to evaluate clinically isolated syndrome (CIS), which is when someone first experiences an attack of neurological symptoms that lasts more than 24 hours. A CIS episode usually has symptoms that are characteristic of an MS relapse. It may consist of a single symptom (a monofocal episode) or more than one symptom (a multifocal episode).

Similar to an MS relapse, neurological symptoms from CIS will improve or go away entirely. CIS is associated with a risk of MS, but it may not result in a diagnosis of MS if other criteria are not met.

Testing for MS With the McDonald Criteria

If someone experiences CIS, the next step would be to look for objective evidence of MS in the form of brain lesions and spinal cord lesions. Diagnosing MS with the McDonald criteria requires an MRI, sometimes with an additional analysis of cerebrospinal fluid (CSF).

The McDonald criteria measure how many neurological attacks have happened and how many lesions have been found. Doctors will look for various combinations of these factors:

• Dissemination of lesions in space (DIS) — An MRI scan shows that lesions have occurred in more than one place.
• Dissemination of lesions in time (DIT) — MRI scans show that lesions have developed during more than one incident.
• Oligoclonal bands — These signal the presence of a protein in the CSF that can indicate CNS inflammation.

Testing the CSF for oligoclonal bands requires a lumbar puncture (spinal tap), in which, after local anesthesia is applied, a hollow needle is inserted into the spinal cavity in the lower back so that the fluid can be collected and tested. Oligoclonal bands with immunoglobulin G (IgG) antibodies are found in more than 95 percent of people with MS. However, oligoclonal bands also appear in other diseases, so their presence alone is not enough to diagnose MS.

MS can sometimes be diagnosed after one attack of CIS. But if the criteria are not met, diagnosis can be delayed. In many cases, people may experience two or more attacks of neurological symptoms over time before an MS diagnosis can be confirmed.

How Effective Are the McDonald Criteria in Diagnosing MS?

The McDonald criteria have proved effective in reducing the time it takes to confirm an MS diagnosis. One study of 325 people with CIS — a first episode of neurological symptoms — showed that 66 percent were definitively diagnosed with MS using the 2017 revised McDonald criteria. In comparison, only 42 percent of those people would have been diagnosed with MS using the previous McDonald criteria from 2010.

An important development in the 2017 MS McDonald criteria update is that oligoclonal bands in CSF are now considered significant in the diagnosis of MS as to whether a person develops lesions on more than one occasion. This finding has allowed a higher rate of confirmed MS cases after a first episode.

McDonald Criteria Shortcomings

Despite improvements in the earlier diagnosis of MS, the McDonald criteria have some limitations.

One concern is that diagnosing MS after CIS — a single incident of neurological symptoms — may be premature in some cases and may result in a false positive diagnosis. Some people may meet all the McDonald criteria for MS after experiencing a single episode that includes the formation of lesions, and yet they may never have another attack. The risk is that a false positive diagnosis of MS would likely result in unnecessary treatment.

The McDonald criteria also may not work for unusual cases of CIS, in which the first neurological abnormalities do not appear as typical MS symptoms. Atypical cases of MS may be overlooked, delaying diagnosis.

The McDonald criteria may not account for signs in brain MRI scans for people with medical conditions such as migraine, which in rare instances can look like MS lesions. Likewise, some people test negative for oligoclonal bands initially but later test positive, suggesting a follow-up lumbar puncture may be needed when a diagnosis is hard to confirm.

How the Diagnostic Criteria for MS Might Evolve

Neuroscience researchers have suggested that more sensitive testing for MS is still needed to confirm diagnosis earlier and improve health care for people with MS. A position paper in The Lancet Neurology recommends some of the ways the McDonald criteria might evolve. These include screening for optic nerve damage, adding more advanced imaging techniques, and potentially checking other biomarkers that can identify signs of MS in body tissue or fluids. It’s also important to develop a better understanding of whether current MS diagnostic criteria work effectively for people of diverse racial and ethnic backgrounds.

Other Tests Used in the Diagnosis of MS

Along with the McDonald criteria, your doctor may conduct other tests if a diagnosis of MS is in question. Although no blood test can diagnose MS, blood tests can help determine if your symptoms are due to another disease. An evaluation of electrical signals in the CNS may be conducted with a sensory evoked potentials test, which uses electrodes to measure nerve responses in legs and arms.

If you have concerns about your diagnosis or a delayed diagnosis of MS, be sure to discuss your questions with your neurologist. The diagnostic process can be emotionally stressful, but your doctor can help clarify your next steps.

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