Background: Diagnosing patients with anti-neutrophil cytoplasmic antibody (ANCA) associated vasculitis (AAV) can be challenging due to its rarity, complexity and wide variety of symptoms. Diagnostic delay may lead to delayed treatment potentially leading to progressive disease and chronic damage. Few studies have addressed real-life diagnostic pathways to identify opportunities to improve the diagnostic phase for AAV patients. Therefore, we evaluated the diagnostic phase of AAV patients in the Netherlands. Methods: This study is a retrospective, observational study of electronic medical records data in hospitals focusing on diagnostic procedures during the first assessment until diagnosis. Results: 230 AAV patients in 9 Dutch hospitals diagnosed with mainly granulomatosis with polyangiitis (73%) and generalized disease (72%) with major organ involvement (kidney, heart, lungs and nervous system) were included. First assessments upon hospital presentation was performed by a specialist in internal medicine (including nephrology) (52%), pulmonology (14%), ear-nose-throat (ENT; 13%) and rheumatology (10%). The median time to diagnosis after referral was 13 days [IQR 2-49] with a difference between patients with generalized and non-generalized disease (9 days [IQR 1-43] vs 22 days [IQR 3-73], p=0.094). The median time to diagnosis after referral in patients with their first assessment by a specialist from internal medicine was 6 days [1-25], rheumatology 14 days [4-45], pulmonology 15 days [5-70] and ENT 57 days [16-176] (p=0.004). A total of 219 biopsies were performed in 187 patients (81%). Histopathological support for AAV diagnosis was observed in 86% of kidney biopsies (84/98), 64% of lung biopsies (14/22), 34% in ENT biopsies (21/61) and 30% of skin biopsies (7/23). Conclusion: In the Netherlands, AAV is predominantly diagnosed and managed by specialists from internal medicine. Diagnostic delay was associated with non-generalized disease and ENT-involvement as presenting symptom. Additionally, ENT biopsies had a ...