Anti-MPO447–461 autoantibody is detectable in 53% of patients tested by ELISA (n=27/51), occasionally at disease onset. (A) Measured serum or plasma anti-MPO447–461 ELISA reactivity from healthy controls (HC), patients with PR3-ANCA vasculitis, patients with systemic lupus erythematous (SLE), and patients with MPO-ANCA vasculitis, normalized to a positive control. Threshold of positivity was determined by the average of healthy control samples plus two standard deviations and is indicated by the grey dashed line. P-values calculated using Wilcoxon two sample test with a Bonferroni adjusted critical value of 0.0083. (B) Reactivity of 20 samples collected at onset paired with a remission sample from the same patient, normalized to a positive control. P-value calculated using matched-pairs sign rank test (C) Analysis of onset samples revealed that anti-MPO447–461 negative samples had a higher prevalence of treatment prior to collection. P-values calculated using Wilcoxon two sample test. (D) Representative patient where temporal analysis of patient anti-MPO447–461 reactivity correlates with disease activity (BVAS score).