In particular for follow-up ANCA testing in AAV patients in order to predict a clinical relapse there are multiple open issues: is it possible at all, for which patients this applies best (cf, MPO- vs. PR3-ANCA; limited vs. generalized AAV; primary small vessel vasculitis vs. drug-induced vasculitis), how is an ANCA-rise defined, which type of assay is to be used, do we need alternative IQC and EQC, and is harmonization feasible for this purpose. Here, MPO is linked to anti-neutrophil cytoplasmic antibody-associated vasculitis.