SEASONALITY HAS NO ASSOCIATION WITH TIMING OF DIAGNOSIS OF RENAL ANCA ASSOCIATED VASCULITIS

Abstract Background and Aims Seasonal variation of ANCA associated vasculitis (AAV) and a possible link to extrinsic infective triggers is predominantly based on single centre epidemiological data. Despite frequent confounding factors including difficulty in identifying the precise time of disease onset, seasonality may be associated with the type of vasculitis and may impact the incidence of renal involvement. Therefore, the aim of this study was to explore if there is an association between seasonality, severity and incidence of biopsy-proven renal vasculitis in the Scottish population. Method Using the Scottish renal biopsy registry, we identified all adult native renal biopsies performed across Scotland between 2014 and 2018 with a diagnosis of AAV, including microscopic polyangiitis (MPA) and granulomatosis with polyangitis (GPA). Demographic data including ANCA antibody status, histological diagnosis, estimated glomerular filtration rate (eGFR) and proteinuria at presentation were recorded. Seasons were defined as autumn (September – November), winter (December-February), spring (March – May) and summer (June - August). Statistical analysis was performed using multivariate ANOVA analysis and Student’s t-test in parametric data. Results 339 cases of biopsy proven AAV were identified and included in the analysis. In this cohort, 53% were female with mean patient age of 65.6 years (± 13). Mean estimated glomerular filtration rate (eGFR) at the time of diagnosis of AAV was 32 (± 27.2) mL/min/1.73m2 and median urinary protein creatinine ratio (uPCR) was 146mg/mmol (IQR 79.8 – 271.3). Diagnosis of MPA n=209(62%) was more common than GPA n=130(38%) and patients with MPA were significantly younger at presentation (63.5 ± 13.6 ‘vs’ 67 ± 12.7 years, p = 0.017). Otherwise, these groups did not differ in mean eGFR (MPA 29.6 ± 25.7 ‘vs’ GPA 34.8 ± 27.6 mL/min/1.73m2) or median uPCR (MPA 147, IQR 78.6 – 286.5 ‘vs’ GPA 139, IQR 80.5 – 261 mg/mmol) at onset. We observed a mean of 3.5 (± 1) new cases of MPA and 2.1 (± 0.7) new cases of GPA per month, with no significant difference observed in month-to-month comparison. Seasonal analysis showed mean occurrence of 11.4 (± 4.5) cases of MPA in autumn, 11.2 (± 4.9) in winter, 10.6 (± 1.5) in spring and 8.6 (± 1.9) in summer months. In GPA, mean 6.6 (±2.7) cases occurred each autumn, 5.4 (± 3) in winter, 7.2 (± 2.9) in spring and 6.4 (± 0.9) in summer months. Overall, no significant differences in monthly or seasonal incidence across 5 years of monitoring were detected. Similarly, we observed no difference in renal function at presentation during different seasons for MPA (mean eGFR range 21.8 – 37.6 mL/min/1.73m2, uPCR median range 112 – 167.5 mg/mmol) or GPA (mean eGFR range 32-37 mL/min/1.73m2; uPCR median range 110 – 244 mg/mmol). Conclusion Our data suggest that there is no seasonal variation in the incidence of AAV diagnosed on kidney biopsy in patients living in Scotland. Additionally, patients present with similar levels of kidney function regardless of season. Thus traditional holiday periods i.e. Easter/Christmas do not seem to lead to a delay in diagnosis. This is the first study to consider seasonality in a complete national cohort and suggests that seasonal extrinsic factors do not play a major role in the pathogenesis leading to AAV onset.