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Objective: To evaluate clinical links between levels of myeloperoxidase (MPO)-antineutrophil cytoplasmic antibody (ANCA) and relapse in patients with ANCA-associated vasculitis (AAV) using a data set from 2 nationwide prospective cohort studies., Methods: From the cohort studies, MPO-ANCA-positive patients who achieved remission during the 6 months after remission induction therapy were enrolled. We measured MPO-ANCA levels at months 0, 3, 6, 12, 18, 24, and at the time of relapse. The primary outcome measure was relapse. A nested case-control analysis and multivariable analysis were performed to investigate the relationship between ANCA reappearance and relapse., Results: Of 271 patients, 183 were classified as having microscopic polyangiitis, 34 as having granulomatosis with polyangiitis, 15 as having eosinophilic granulomatosis with polyangiitis, and 39 were unclassifiable. The median age was 73 years, and 165 (61%) were female. In 195 patients (72%), MPO-ANCA levels decreased to normal levels within 6 months after commencement of treatment, and MPO-ANCA reappeared in 73 of 181 patients (40%) with complete follow-up data. Reappearance of MPO-ANCA was more frequent in patients with relapse than in 75 age- and sex-matched control patients without relapse (odds ratio 26.2 [95% confidence interval 8.2-101], P < 0.0001) after adjustment for confounding factors., Conclusion: Reappearance of MPO-ANCA could be a clinically useful biomarker for predicting relapse in patients with MPO-ANCA-positive AAV in remission. This suggests that routine MPO-ANCA monitoring should be implemented in this patient population., (© 2018, American College of Rheumatology.)