Background Treatment with dimethyl fumarate (DMF) may be associated with drug-induced lymphope- nia. The Summary of Product Characteristics recommends monitoring lymphocyte counts 3-monthly, as part of a risk mitigation strategy for progressive multifocal leukoencephalopathy. In February 2020, the COVID-19 pandemic led to a reduced frequency of blood monitoring to 6-monthly in stable patients on DMF (>12 months of treatment, lymphocyte count >0.6x109).

Aim To investigate the impact of reduced frequency of DMF blood monitoring in detection of lymphopenia.

Methods A retrospective chart review was performed of MS patients treated with DMF since August 2015. Lymphocyte counts and demographic information was recorded. Data was analysed to determine if reduced frequency of blood monitoring led to any cases of severe lymphopenia (<0.5x109) being missed.

Results 184 DMF-treated patients were included; 120 stable patients had reduced frequency of blood monitoring during the pandemic, for a median follow-up of 15 months. Only one patient was identified a lymphocyte count <0.5 x109 during 6-monthly blood monitoring, but this was not sustained, and treatment with DMF was continued.

Conclusion Monitoring lymphocyte counts 6-monthly does not significantly increase the risk of missing severe lymphopenia in patients stable on treatment with DMF.