# Dissecting definitions of disability accrual in relapsing multiple sclerosis—Have we reached standardization yet?

## TL;DR

This study examines how different definitions of disability progression in multiple sclerosis affect reported outcomes, highlighting inconsistencies in current methods.

## Contribution

The paper evaluates 1440 definitional variants of disability accrual in RMS using a large registry dataset to quantify variability in event rates and RAW/PIRA proportions.

## Key findings

- Disability accrual rates varied between 15.7% and 41.6% depending on the definition used.
- Progression independent of relapse activity (PIRA) accounted for 56.1%–86.3% of events across definitions.
- Even with 'standardized' criteria, 144 definitional combinations remained, leading to wide variability in event rates and PIRA contributions.

## Abstract

Distinguishing relapse-associated worsening (RAW) and progression independent of relapse activity (PIRA) has reshaped understanding of disability accumulation in relapsing multiple sclerosis (RMS). The influence of differing definitions of disability accrual on event rates and RAW/PIRA proportions remains uncertain.

This observational cohort study used Austrian MS Treatment Registry data (2010–2024). A custom algorithm evaluated 1440 definitional variants of disability accrual with varying confirmation duration, baseline modeling, and RAW/PIRA classification, including recently proposed “standardized” criteria.

We included 3273 RMS patients (mean age 37.5 years; 67.8% female) with ⩾24 months follow-up, ⩾3 Expanded Disability Status Scale (EDSS) scores, and ⩾1 EDSS score per year, contributing 3525 follow-up periods. Depending on definition, disability accrual varied between 15.7% and 41.6% of follow-ups. PIRA accounted for 56.1%–86.3% of events across definitions, while up to 8.4% were ambiguously classified, mainly due to post-relapse re-baselining or relapses during the confirmation period. Even under “standardized” criteria, 144 definitional combinations remained, with event rates ranging from 19.1% to 21.7% and PIRA contribution varying widely from 59.8% to 85.8%.

PIRA predominantly drives disability accrual, yet definitional variation substantially influenced event rates and RAW/PIRA proportions. Transparent reporting and further optimization of definitions are critical for improving comparability, interpretation, and clinical relevance in MS research and care.