Patient-reported outcomes are the strongest predictors of disease disability in intramuscular interferon beta-1a users.

Introduction: Patient-reported outcomes (PROs) are valuable measures for routine clinical care of people with multiple sclerosis (pwMS). Materials: 646 pwMS treated with interferon-β-1a (IFN-β-1a) were retrospectively included from the New York State Multiple Sclerosis Consortium. Clinical and PRO data at enrollment and 3 year follow-up were collected. PwMS with stable disease and disability worsening were matched (1:1) based on age, Expanded Disability Status Scale (EDSS) scores and disease duration. Disability worsening was determined based on trial criteria. Results: PwMS with future EDSS worsening had higher baseline and follow-up timed-25-foot walk (6.6 vs 5.5 s; 9.1 vs 5.5 s; p < .001) when compared with stable pwMS. Worsening pwMS reported higher baseline difficulties in getting up (odds ratio [OR] = 2.4; p = 0.009), climbing stairs (OR = 1.6; p = 0.024) and standing (OR = 2.2; p < 0.001). Worsening pwMS reported greater lower limb limitations (OR = 2.3; p = 0.004) and fatigue (OR = 1.8; p = 0.002). Conclusion: Higher fatigue and lower limb functional limitations are significant predictors of future disability worsening in pwMS.