Objectives: We investigated whether systemic immune-inflammation index (SII) at diagnosis can estimate the cross-sectional high activity and predict the poor outcomes in immunosuppressive drug-naïve patients with antineutrophil cytoplasmic antibody-associated vasculitis (AAV). Methods: We retrospectively reviewed the medical records of 163 patients with AAV and obtained clinical and laboratory data. We calculated Birmingham vasculitis activity score (BVAS) as well as five-factor score (FFS) (2009) at diagnosis. SII at diagnosis was calculated by the equation of (SII at diagnosis = platelet count × neutrophil count/lymphocyte count at diagnosis). Severe AAV was defined as BVAS at diagnosis ≥16. The odds ratio was assessed using the multivariable logistic regression analysis and cumulative survival rates were compared by the Kaplan–Meier survival analysis. Results: The median age at diagnosis was 58.0 years old and 51 patients were men. The median BVAS was 12.0. Fifty-seven patients had severe AAV. The median SII at diagnosis was 1349.6. In the multivariable analysis, only SII exhibited a significant odds ratio for the cross-sectional severe AAV (P = 0.043). We obtained the cut-off of SII at diagnosis for severe AAV as 1573.56. Patients with SII at diagnosis ≥1573.56 exhibited a significantly high relative risk of the cross-sectional severe AAV compared to those without (relative risk 4.625). Furthermore, patients with SII at diagnosis ≥1573.56 exhibited significantly the lower cumulative relapse free and renal survivals than those without. Conclusion: Systemic immune-inflammation index at diagnosis could estimate the cross-section severe AAV and predict the poor outcomes in AAV patients.
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