Prevalence of disability improvement as a potential outcome for multiple sclerosis trials.

BACKGROUND: The concept of improvement of disability recently emerged as a new target in multiple sclerosis (MS) studies since the approval of new potent drugs and for testing drugs for neuroprotection and repair. OBJECTIVE: To propose a simple estimator for assessing and comparing the prevalence of improvement over time between groups. METHODS: The prevalence of a transient condition takes into account the incidence and the duration of such condition. We propose here the application of a modified Kaplan-Meier estimator to evaluate and compare between groups the prevalence of improvement over time in a cohort of 121 patients treated with autologous hematopoietic stem cell transplantation. RESULTS: The prevalence of improvement after 5 years from transplant was 50.3% (95%CI: [38.0-63.0]) in relapsing-remitting patients and 6.5% (95%CI: [0-17.8]) in secondary-progressive patients (p < 0.001). Such a difference wouldn't be evident considering the traditional cumulative probability of improvement at 5 years (55.5% in relapsing-remitting vs 33.4% in secondary-progressive patients, p = 0.10). CONCLUSION: This study shows the relevance of a new estimator of prevalence of improvement in MS. This estimator gives simple information on whether a drug can induce a durable improvement in disability and can be considered a potential outcome for trials assessing drugs for neuroprotection or repair.