MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: This study was motivated by the overwhelmingly lower positivity rate of IIF-ANCA tests performed at the Immuno-Rheumatology Laboratory at a reference university hospital compared to IIF-ANCA and PR3- and MPO-ANCA tests performed in a CAP-certified private laboratory in the investigation of AAV and other diseases associated with ANCA.