Antimyeloperoxidase and proteinase 3 antibodies for nephritis flare prediction in ANCA-associated-vasculitis

Background: The value of myeloperoxidase and proteinase 3 antibodies titers in the assessment of renal disease activity and flare prediction in patients with ANCA-associated-vasculitis (AAV) is not well-known. Methods: Retrospective study including 113 AVV patients with a renal biopsy-proven pauci-immune necrotizing glomerulonephritis from seven Spanish hospitals. The main inclusion criteria were assessment of MPO antibodies (MPOab) using multiplex flow immunoassay and PR3 antibodies (PR3ab) measurements using immunoassay chemiluminescence with an identical range of values for all participating centers. Results: Serum MPOab, 3 ± 1.2 months before relapse, was higher in patients who relapsed (19.2 ± 12.2 vs 3.2 ± 5.1 AI, p < 0.001). The discrimination value of MPOab 3 months before renal relapse had an AUC of 0.82 (95%CI 0.73-0.92; p < 0.001). ΔMPOab (change in antibodies titration 6 months before relapse) was higher in patients who relapsed [8.3 ± 12 vs 0.9 ± 3.1 AI, p = 0.001) (AI; antibody index unit). The discrimination value of ΔMPO had an AUC of 0.76 (95%CI 0.63-0.88; p < 0.001). The positive predictive value of renal relapse in PR3 patients is 100% and the negative predictive value of renal relapse in patients with PR3 positive titers is 57.1%. Serum PR3ab was higher in patients who relapsed 2.8 ± 1.4 months before relapse (58.6 ± 24.6 vs 2.0 ± 0.6 AI, p < 0.001). Conclusions: MPO antibody level monitorization using multiplex flow immunoassay and PR3 measurements using immunoassay chemiluminescence are useful and sensitive tools for the prediction of renal relapse in the follow-up of AAV patients with renal disease, and relevant surrogate markers of renal disease activity.