Development and validation of a disease extent index for Wegener's granulomatosis

Aims: The quantitative assessment of disease extent (staging) and activity (grading) in patients with ANCA-associated vasculitides is one prerequisite for the comparison of outcomes in different patient cohorts. We present a validation study of the Disease Extent Index (DEI). Methods: 66 patients with Wegener's granulomatosis were examined to assess the validity, reliability and sensitivity to change of the DEI. Correlation coefficients were calculated to estimate associations between DEI and BVAS (Birmingham Vasculitis Activity Score), a previously established activity score, and between DEI and serological markers. Results: Among patients with active disease, DEI correlated significantly with cANCA titer (r = 0.46), leukocyte count (r = 0.38) and platelet count (r =0.53). Among patients in remission, DEI correlated significantly with cANCA titer (r =0.61), CRP (r = 0.47) and sIL2R (r = 0.47). Additionally, a high association of DEI and BVAS (r = 0.9) confirmed a rather high convergent validity. The effect size of DEI equals 2.37 standard deviation units and indicates a considerable change from active disease to remission. Conclusions: The DEI is valid, reliable, easy, quick to perform and highly reproducible. Although it correlates with some surrogate markers of disease activity and the BVAS, the DEI quantifies different domains of the disease than the BVAS and should therefore be used in conjunction with the BVAS.