The development and initial validation of IgG4-related disease damage index: a consensus report from Chinese IgG4-RD Consortium

Objective To develop and conduct an initial validation of the Damage Index for IgG4-related disease (IgG4-RD DI).Methods A draft of index items for assessing organ damages in patients with IgG4-RD was generated by experts from the Chinese IgG4-RD Consortium (CIC). The preliminary DI was refined using the Delphi method, and a final version was generated by consensus. 40 IgG4-RD cases representing four types of clinical scenarios were then selected, each with two time points of assessment for at least 3 years of follow-up. 48 rheumatologists from 35 hospitals nationwide were invited to evaluate organ damage using the CIC IgG4-RD DI. The intraclass correlation coefficient (ICC) and the Kendall-W coefficient of concordance (KW) were used to assess the inter-rater reliability. The criterion validity of IgG4-RD DI was tested by calculating the sensitivity and specificity of raters.Results IgG4-RD DI is a cumulative index consisting of 14 domains of organ systems, including a total of 39 items. The IgG4-RD DI was capable of distinguishing stable and increased damage across the active disease subgroup and stable disease subgroup. In terms of scores at baseline and later observations by all raters, overall consistency in scores at baseline and later observations by all raters was satisfactory. ICC at the two time points was 0.69 and 0.70, and the KW was 0.74 and 0.73, respectively. In subgroup analysis, ICC and KW in all subgroups were over 0.55 and 0.61, respectively. The analysis of criterion validity showed a good performance with a sensitivity of 0.86 (95% CI 0.82 to 0.88), a specificity of 0.79 (95% CI 0.76 to 0.82) and an area under the curve of 0.88 (95% CI 0.85 to 0.91).Conclusion The IgG4-RD DI is a useful approach to analyse disease outcomes, and it has good operability and credibility. It is anticipated that the DI will become a useful tool for therapeutic trials and studies of prognosis in patients with IgG4-RD.