Incidence of anti-neutrophil cytoplasmic antibody-associated renal vasculitis: a retrospective study in rural and regional Victoria, Australia.

Background: The epidemiology of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) varies worldwide. Previous Australian studies described a higher incidence of AAV in rural areas; however, this has not yet been investigated in Victoria.

Aims: To calculate the incidence of AAV in rural and regional Victoria and characterise the demographics and clinical outcomes of this cohort.

Methods: We performed a retrospective review of patients with newly diagnosed AAV confirmed on renal biopsy at Bendigo Health between 2013 and 2021. Cases were classified according to the 2022 American College of Rheumatology/European Alliance of Associations for Rheumatology criteria. Local disease incidence was calculated using Estimated Resident Population data for our catchment, the Loddon Mallee region.

Results: Twenty-eight cases of new AAV were identified; 17 were classified as microscopic polyangiitis (MPA) and the remainder as granulomatosis with polyangiitis (GPA). The median age at diagnosis was 68 years (interquartile range (IQR): 59-77). The incidence per million person-years was 9.3 for AAV overall (95% CI: 6.2-13.5), 5.7 for MPA (95% CI: 3.3-9.1) and 3.7 for GPA (95% CI: 1.8-6.6). With a median follow-up time of 3.3 years (IQR: 1.9-5.6), one-quarter of patients relapsed (n = 7, 25%), and six required ongoing renal-replacement therapy (21%).

Conclusions: The calculated incidence of AAV in rural and regional Victoria is not higher than the reported incidence in most urban Australian cohorts. This study may underestimate the true local disease incidence as only patients with renal vasculitis were included.