Background and purpose Dimethyl fumarate (DMF) causes a mean lymphocyte count drop of approximately 30% in relapsing-remitting multiple sclerosis (RRMS) patients. The relationship between this reduction and DMF effectiveness is controversial. The objective was to investigate if the decrease in absolute lymphocyte count (ALC) from baseline during DMF treatment is associated with clinical and magnetic resonance imaging (MRI) disease activity. A secondary aim was to evaluate ALC variations over time in a real-life cohort of DMF-treated patients. Methods Demographic, laboratory, clinical and MRI data were collected in this observational multicentre study, conducted on RRMS patients treated with DMF for at least 6 months. Multivariate Cox models were performed to evaluate the impact of 6-month ALC drop on time to no evidence of disease activity (NEDA-3) status loss. NEDA-3 is defined as absence of clinical relapses, MRI disease activity and confirmed disability progression. Results In all, 476 patients (312 females, age at DMF start 38.4 +/- 9.97 years) were analysed up to 5-year follow-up. A greater lymphocyte decrease was associated with a lower risk of NEDA-3 status loss (hazard ratio 0.87,P = 0.01). A worse outcome in patients with lower ALC drop (<11.5%), compared with higher tertiles (11.5%-40.5% and >40.5%), was observed (P = 0.008). The nadir of ALC drop (-33.6%) and 35% of grade III lymphopaenia cases occurred after 12 months of treatment. Conclusion A higher lymphocyte count drop at 6 months is related to better outcomes in DMF-treated patients. A careful ALC monitoring should be pursued up to 24 months of treatment.
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