Unclassifiable repeated antineutrophil cytoplasmic antibody (ANCA) positivity in diseases other than ANCA-associated vasculitis

Background and objective Antineutrophil cytoplasmic antibody (ANCA) is a specific autoantibody for ANCA-associated vasculitis (AAV). However, ANCA can be detected in various diseases other than AAV. Hence, in this study, we investigated and provided the name of diseases with repeated ANCA positivity and the frequency of each disease other than AAV. Methods We retrospectively screened the results of the tests of ANCA in 26,499 patients using the Clinical Data Repository System and included in this study only 173 patients with repeated ANCA positivity more than twice. ‘Unclassifiable ANCA’ was defined when ANCA was detected in patients with diseases other than AAV. ‘Unclassifiable repeated ANCA’ was also defined when unclassifiable ANCA was successively detected more than twice. Results Among rheumatic and autoimmune diseases, the most common disease with unclassifiable repeated ANCA was vasculitis undetermined (21.0%). In terms of cardiovascular and cerebrovascular diseases, the most common disease with unclassifiable repeated ANCA was atherosclerotic heart disease (12.1%). In terms of disorders in liver, kidneys and lungs, the most common disease with unclassifiable repeated ANCA was chronic kidney disease (51 cases, 29.5%). In addition, among infections with confirmed infectious pathogens, the most common pathogen with unclassifiable repeated ANCA was varicella-zoster virus (6.9%) followed by Candida (4.6%). Conclusion Overall, regardless of category, the common diseases with unclassifiable repeated ANCA were chronic kidney disease followed by interstitial lung disease and vasculitis undetermined. Thus, we carefully suggest that physicians should pay more attention to the development of AAV or vasculitis other than AAV and, furthermore, kidneys and lungs should be monitored regularly and closely in patients with unclassifiable repeated ANCA.