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Background and Objectives: Brief cognitive tests to monitor cognitive impairment in patients with multiple sclerosis (MS) are needed.Methods: Performance on monthly administrations of the Symbol Digit Modalities Test (SDMT) and the MS Neuropsychological Questionnaire (MSNQ) was assessed in 660 patients with MS in 21 countries (109 sites) for 48 weeks in an open-label, safety-extension study of natalizumab.Results: At baseline, the cohort’s mean age was 40.1 years, 67.6% were female and the median Expanded Disability Status Scale score was 2.5. Test—retest correlations were high for both SDMT (range 0.89 for weeks 0—4 to 0.96 for weeks 44—48) and MSNQ (0.82 for weeks 0—4 to 0.93 for weeks 44—48). There were no statistically significant effects of geographic region. While SDMT scores improved by 15 points over 48 weeks (p < 0.0001), incremental monthly changes were small (effect size d < 0.3). Similar results were obtained on the MSNQ except that scores moved downward, suggesting fewer cognitive complaints over 48 weeks (p < 0.0001), but again the incremental monthly changes were small (d <-0.2).Conclusions: These results replicate earlier work in a smaller cohort treated with conventional disease-modifying therapy, and support the reliability of the SDMT and MSNQ as potential screening for monitoring tools for cognition over time. [ABSTRACT FROM PUBLISHER]