Diagnosing multiple sclerosis: implementation and unintended effects of the 2024 McDonald criteria.

Diagnostic criteria for multiple sclerosis have evolved substantially since the Schumacher criteria were published in 1965. The most recent iteration, the 2024 McDonald diagnostic criteria, was recently released. Each revision has broadened the range of evidence that can support diagnosis while attempting to preserve specificity. The central question raised by the 2024 revision is therefore not simply whether the criteria are valid in principle, but whether they can be applied safely outside expert centres, where imaging quality, laboratory infrastructure and subspecialist interpretation vary substantially. In this review, we evaluate the 2024 criteria not only as a consensus document but as an intervention in clinical behaviour, with particular attention to misdiagnosis risk, operational feasibility and equity of implementation. In particular, we consider the role of the optic nerve and of the central vein sign and paramagnetic rim lesions in diagnosis, and how the criteria bring more diverse populations and presentations into consideration. We also review how the greater options available to achieve a diagnosis may introduce heterogeneity into diagnostic approaches, and what safeguards can be taken to minimise the risk of inappropriate application of the criteria and subsequent misdiagnosis (and inappropriate treatment). The review finishes by proposing how the changes introduced in this iteration might be refined, with more robust definitions on acquiring and interpreting the novel biomarkers.