Lower Myeloperoxidase-ANCA Titres at Diagnosis Are Associated with End-Stage Kidney Disease Progression During Follow-Up in Rituximab-Treated Patients with Microscopic Polyangiitis

<i>Background and Objectives</i>: To investigate whether myeloperoxidase (MPO)-antineutrophil cytoplasmic antibody (ANCA) titres at diagnosis are associated with the risk of end-stage kidney disease (ESKD) progression in patients with microscopic polyangiitis (MPA) treated with rituximab. <i>Materials and Methods</i>: This retrospective cohort study included 34 patients with MPA who received rituximab. Clinical data, including MPO-ANCA titres at diagnosis and ESKD progression during follow-up, were assessed. Receiver operating characteristic (ROC) curve analysis was performed to assess whether MPO-ANCA titres could predict ESKD progression. The optimal cut-off value of MPO-ANCA titres was determined where the sum of sensitivity and specificity was at a maximum. Based on this cut-off value, patients were categorised into two groups, and the relative risk (RR) of ESKD progression was estimated. <i>Results</i>: During a median follow-up of 39.5 months, seven patients (20.6%) progressed to ESKD. ROC curve analysis showed a significant inverse association between MPO-ANCA titres and ESKD progression (AUC 0.254, 95% confidence interval [CI] 0.046, 0.462 <i>p</i> = 0.048). The optimal cut-off of MPO-ANCA titres was 81.0 IU/mL, which yielded a sensitivity and specificity of 70.4% and 85.7%, respectively. The RR of ESKD progression was significantly higher in those with MPO-ANCA titres ≤ 81.0 IU/mL than in those with MPO-ANCA titres > 81.0 IU/mL (42.9% vs. 5.0%, RR 14.250, 95% CI 1.469, 138.271). <i>Conclusions</i>: Lower MPO-ANCA titres at diagnosis may be associated with a higher risk of ESKD progression in rituximab-treated MPA patients. These findings suggest that MPO-ANCA titres may be useful in guiding therapeutic decisions for MPA.