Early predictive risk factors for dimethyl fumarate-associated lymphopenia in patients with multiple sclerosis.

• Lymphocyte count decreases along the first two years of dimethyl fumarate therapy. • One-third of patients on dimethyl fumarate will develop some degree of lymphopenia. • Older age and lower baseline lymphocyte count increase the risk of lymphopenia. • Early and pronounced decline in lymphocyte count is the best predictor of lymphopenia. Lymphopenia is a major concern in MS patients treated with dimethyl-fumarate (DMF) as it increases the risk of progressive multifocal leukoencephalopathy. A pronounced reduction in absolute lymphocyte counts (ALCs) early after treatment initiation has been suggested to be associated with the occurrence of lymphopenia thereafter. To identify risk factors for DMF-induced lymphopenia and evaluate whether the degree of decrease in the ALCs three months after initiation of DMF treatment is a predictor of the subsequent development of lymphopenia. In this real-world Spanish prospective multicenter study conducted in MS patients who started DMF between 2014 and 2019, we analyzed the association between DMF-related lymphopenia and the percentage of early ALCs decline using regression models, considering both, significant lymphopenia (grades 2 + 3) and severe lymphopenia (grade 3). The cutoff values of early ALCs declines were obtained using the ROC curve. Among 532 MS patients treated with DMF, 193 (36.3%) developed any grade of lymphopenia. Older age and lower ALCs at treatment onset predicted the risk for lymphopenia but the best predictive risk factor was the reduction of ALCs within the three first months of treatment. Specifically, a reduction in ALCs≥21.2% was associated with a 6.5-fold higher risk of developing significant lymphopenia, and a decrease in ALCs≥40.2% with a 12.7-fold higher risk of developing severe lymphopenia. A pronounced reduction in ALCs early after initiation of DMF in MS patients is the best predictive risk factor for the subsequent development of significant lymphopenia. [ABSTRACT FROM AUTHOR]