The avidity of PR3-ANCA in patients with granulomatosis with polyangiitis during follow-up

The objective of this study is to investigate whether the avidity of proteinase-3-anti-neutrophil cytoplasmic antibody (PR3-ANCA) changes during follow-up in different subgroups of patients with granulomatosis with polyangiitis (GPA). We selected 10 patients with renal relapsing GPA, 10 patients with renal non-relapsing GPA and 10 patients with non-renal relapsing GPA. In all patients, an ANCA rise occurred during remission. The avidity was measured using a chaotropic approach at the time of an ANCA rise and at the time of a relapse in relapsing patients or time-matched during remission in non-relapsing patients. No difference was observed in the avidity at the ANCA rise between renal relapsing patients [262% (155-475)], renal patients without a relapse [396% (212-634)] and non-renal relapsing patients [342% (216-595)]. In renal relapsing patients, the avidity increased significantly from the moment of the ANCA rise to the relapse [difference 64% (00-171), P=00273]. The avidity did not increase after an ANCA rise in renal non-relapsing patients [difference 35 (-60 to 101), P=06250] or in non-renal relapsing patients [difference -31% (-80 to 50), P=05703]. The avidity of PR3-ANCA increases after an ANCA rise during follow-up in renal relapsing patients, but not after an ANCA rise in renal patients who remain in remission or in non-renal relapsing patients.