Randomized clinical trials evaluate short-term treatment effect on disability in the form of 3-6-month confirmed disability progression, but whether these translate into long-term disability outcomes is unknown. 

In 11,435 confirmed progression events identified in 6,902 patients (development cohort), patient characteristics at the time of progression associated with lower probability of subsequent improvement were age (hazard ratio (HR)=0.98), primary progressive (HR=0.37) and progressive-relapsing (HR=0.36) MS, expanded disability status scale score >=6 (HR=0.71) and its change from baseline (HR=0.67), number of affected functional system scores (HR=0.92) and pyramidal (HR=0.79) functional system score (p<0.05). The strength of the association with pyramidal score decreased with time (HR=1.01). A relapse within previous month (HR=1.46) and worsening in sensory functional system score (HR=1.17) were associated with higher probability of improvement after progression. The sustained progression score (range 0.39-4.79) in the validation cohort with 1,271 progression events, estimated a 53% lower chance of improvement for each unit increase in the score (HR=0.47). The proportions of progression events sustained at 5 years stratified by the score were 1: 68%, 2: 79%, 3: 94%, 4: 100%. The progression scores were then applied to the CLARITY trial data.

Estimate of the probability of disability progression events being sustained over the long term will enable future trials to establish the effect of therapy not only on the short-term but also on long-term disability accrual.