Background: Joint damage is an important outcome in trials of RA, usually assessed by Total Sharp Score (TSS). It is currently unknown how it translates numerically into disability by the health assessment questionnaire (HAQ).
Objective: To determine the units of HAQ score corresponding to one TSS unit.
Methods: We evaluated a short-term observational trial of glucocorticoids in RA (BELIRA), and used randomized controlled clinical trial (RCT) data for confirmation. For each trial arm we assessed HAQ, TSS, and the simplified disease activity index (SDAI). Based on the hypothesis that short term HAQ changes will mostly be due to changes of disease activity, we determined activity-HAQ (ACT-HAQ) at endpoint (EP), and defined remaining disability as damage-related (DAM-HAQ). Using TSS at EP, we estimated the HAQ units corresponding to a TSS unit.
Results: In BELIRA, one TSS unit corresponded to a mean of 0.017 HAQ units; to account for other causes of irreversible disability, we used the 25th percentile: 0.011 HAQ units/TSS unit. In RCT trial arms, the HAQ/TSS were similar (0.013 and 0.015 in established and early RA, respectively; 25th percentile: 0.010). The correlation between DAM-HAQEP and TSS was r=0.829. Over 5 years, damage would amount to an increase of irreversible HAQ of 0.33 on placebo, 0.13 on DMARDs and 0.03 on TNF-inhibitors+MTX.
Conclusion: We presented an approach to estimate the numerical relationship between HAQ and damage as 0.01 HAQ points/TSS unit, although linear relationship may not be generally valid. This allows assessing functional correlates of radiographic changes in trials.