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Titre and affinity of propylthiouracil‐induced anti‐myeloperoxidase antibodies are closely associated with the development of clinical vasculitis

Substantial evidences suggested that propylthiouracil (PTU) could induced anti‐myeloperoxidase (MPO) antibodies in sera from patients with hyperthyroidism, however, only a subgroup of the PTU‐induced anti‐MPO antibody positive patients developed clinical evident vasculitis. The aim of this study is to compare the titres and affinities of PTU induced anti‐MPO antibodies in sera from patients with hyperthyroidism with and without clinical vasculitis. Anti‐MPO antibody positive sera from patients diagnosed hyperthyroidism with (n = 13) and without (n = 14) clinical evident vasculitis were collected. The titre was determined by MPO‐ELISA and expressed as logarithm value (lgT). The affinity constant (aK) of anti‐MPO IgG was measured by antigen inhibition assay. The titre and aK values were compared between patients with and without vasculitis. In patients with vasculitis, the mean lgT of anti‐MPO antibodies was 3·62 ± 0·66; the median aK was 4·47 × 107M−1. In patients without vasculitis, the mean lgT was 2·54 ± 0·29; the median aK was 0·14 × 107M−1, and both were significant lower than those in patients with vasculitis (t = 5·464; P = 0·000 & z = −4·373; P= 0·000, respectively). We concluded that the titre and affinity of anti‐MPO antibodies might be associated with the development of clinical vasculitis in patients with PTU‐induced ANCA.