Incidence of biopsy-proven giant cell arteritis (GCA) in South Australia 2014–2020

Objective: To determine the incidence of biopsy proven giant cell arteritis (GCA) in South Australia. Methods: Patients with biopsy-proven GCA were identified from pathology reports of temporal artery biopsies at state-based pathology laboratories, from 1 January 2014 to 31 December 2020. Incidence rates for biopsy-proven GCA were calculated using Australian Bureau of Statistics data for South Australian population sizes by age, sex, and calendar year. Seasonality was analyzed by cosinor analysis. Results: There were 181 cases of biopsy-proven GCA. The median age at diagnosis of GCA was 76 years (IQR 70, 81), 64% were female. The estimated population incidence for people over 50 was 5.4 (95% CI 4.7, 6.1) per 100,000-person years. The female: male incidence ratio was 1.6 (95% CI 1.2, 2.2). There was no ordinal trend in GCA incidence rates by calendar year (p = 0.29). The incidence was, on average, highest in winter, but not significantly (p = 0.35). A cosinor analysis indicated no seasonal effect (p = 0.52). Conclusion: The incidence of biopsy-proven GCA remains low in Australia. A higher incidence was noted compared to an earlier study. However, differences in ascertainment and methods of GCA diagnosis may have accounted for the change.