Background/Purpose: Idiopathic inflammatory myositis (IIM) significantly impacts health-related quality of life (HRQoL). EQ5D-5L is a widely used and validated tool to measure HRQoL. The health utility scores (HUS) derived from EQ5D-5L are used to calculate QALY and to analyze cost effectivity. To the best of our knowledge, there are no studies that have validated EQ5D-5L in IIM. To test the validity of EQ-5D HUS in assessing HRQoL in IIM. To assess the responsiveness to change and to calculate minimal clinically important differences (MCID) of EQ-5D HUS

Methods: This was a single centre prospective study involving patients with IIM. Patients were evaluated at 0 and 6 months with all 6 myositis core set measures and EQ5D-5L. The EQ5D-5L health states for Indian population were used to calculate HUS. Individuals with MMT8 of < 74/80 and / or those who had extra muscular disease activity of > 2/10 were considered active and the rest as inactive disease. Muscle weakness was classified as severe (MMT-8 < 54/80), mild-moderate (MMT-8 54 to 73/80) and inactive (MMT-8 ≥74). Construct validity of EQ5D-HUS (ability to differentiate different health states) was tested by comparing scores and Cohens’D effect size (ES) across disease severity. Responsiveness to change was estimated with ES for the change in scores between those with improvement and no improvement in HAQ-DI and total improvement scores (TIS). MCID was calculated using distribution-based method (0.5*SD of change) and anchor based method (ROC curve) using a change of ≥0.22 of HAQ-DI between 0 and 6 months as the anchor.

Results: Fifty-eight IIM patients with mean (SD) age of 37.6 (9.8), 76% (N=44) females were included in the study. Disease subset included 34 (58.6%) DM, 18 (31%) anti-synthetase syndrome, 3 (5.2%) immune mediated necrotizing myopathy and 3 (5.2%) PM.

Median MMT-8 (0-80) at baseline was 61.35 ±15.49 with a median (IQR) of extra-muscular disease activity (0-10) of 3 (0,5). The median (range) EQ5D-5L health utility score was 0.415 (-0.69, 0.87). Sixteen patients had severe myositis with EQ-5D HUS of -0.68, 34 mild to moderate had EQ-5DHUS of 0.42 and 8 had inactive disease EQ-5DHUS- 0.81 at baseline (p< 0.01). The changes in MMT-8, HAQ-DI, physician and patient global assessment, extra muscular diseases activity and EQ5-D HUS between 0 and 6 months in the cohort is represented in table 1.

The EQ-5D HUS increased by 0.625 (p < 0.001) in 6 months and was able to differentiate between patients with improvement based on HAQ-DI and 2016 ACR/EULAR Myositis response criteria (TIS ≥20) (Table 1). The ES of change in MMT-8, HAQ-DI, Physician and patient global assessment, extra muscular diseases activity and EQ5-D HUS between those with improvement as per HAQ-DI ≥0.22 and TIS ≥ 20 is in table 2.
Upon calculating the MCID to detect a minimal change in HAQ DI of 0.22, the best model with a sensitivity of 96.9 and specificity of 88.5 was at an EQ-5D HUS of 0.303. The distribution-based MCID calculation using the 0.5 SD method resulted in a value of 0.34 for EQ-5D HUS (Table 3).

Conclusion: Health utility scores derived from EQ5D-5L is a valid instrument for measuring the HRQoL in IIM. An MCID of 0.3- 0.34 had the highest sensitivity and specificity to detect change in clinical state among patients with IIM.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/validity-responsiveness-and-minimal-clinically-important-difference-of-eq-5d-5l-in-inflammatory-myositis-a-longitudinal-study/