RESUMEN
Eosinophilia is common during natalizumab treatment for multiple sclerosis but risk factors are unknown. We aimed to identify demographic, clinical and laboratory characteristics predicting eosinophilia. Sustained eosinophilia occurred in 16.8%. Risk factors for sustained eosinophilia included baseline pre-treatment eosinophilia, medical conditions potentially associated with eosinophilia including allergies, and suboptimal compliance. One temporal profile was associated with the highest and most rapidly developing eosinophilia, and was less likely to resolve: in one such case, eosinophilia was symptomatic. Changes in eosinophil and lymphocyte counts were only weakly correlated, suggesting factors other than Very Late Antigen-4 (VLA-4) inhibition drive eosinophilia.