Clinical Meaningfulness and Improvement Thresholds of Myositis Core Set Measures: Association With Patient-Reported Outcomes.

OBJECTIVE

This study aimed to evaluate the clinical meaningfulness and improvement thresholds of the myositis core set measures (CSMs) of disease activity in idiopathic inflammatory myopathies (IIMs), emphasizing associations with patient-reported outcome measures (PROMs).

METHODS

Adults with IIM were enrolled in a 6-month prospective observational study. PROMs were collected monthly, including patient global assessment (PtGA), Health Assessment Questionnaire- Disability Index (HAQ-DI), pain and fatigue (visual analog scale [VAS]), 36-item Short Form Health Survey (SF-36), and Patient-Reported Outcomes Measurement Information System Physical Function Short Form 20. Physician-reported outcomes (physician global assessment [PGA], extramuscular global disease activity [ExGLB], manual muscle testing 9 [MMT9]) and Total Improvement Score (TIS) of the 2016 American College of Rheumatology/European Alliance of Associations for Rheumatology (ACR/EULAR) Myositis Response Criteria were collected every 3 months. Associations were evaluated using Spearman correlation and linear mixed models to assess construct validity. Responsiveness, effect sizes, and clinically important differences (CIDs) were assessed using the anchors patient-reported and physician-reported global impression of change (PGIC and PhGIC, respectively) at 6 months.

RESULTS

Fifty patients (mean age, 51.6 [SD 14.9] years; 60% female) were enrolled. Most CSMs showed strong baseline and longitudinal associations with PROMs, including pain, fatigue, physical function, and quality of life (QOL). Significant improvements were observed in most CSMs with moderate to large effect sizes, using PGIC and PhGIC as anchors. The CIDs according to PGIC were 2.1 for PGA, 1.4 for ExGLB, 1.7 for PtGA, 4.2 for MMT9, 0.5 for HAQ-DI, 1475 IU/L for creatine kinase, and 28.0 for TIS. PhGIC demonstrated similar results.

CONCLUSION

CSMs demonstrate robust validity and responsiveness, closely aligning with patients' symptoms, function, and QOL and supporting their utility in IIM trials. We established CIDs for CSMs and TIS.