Box 2 Screening for ANCA

Anti-neutrophil cytoplasmic antibodies (ANCA) form a family of autoantibodies directed against antigens contained in the primary granules (or azurophils) of the cytoplasm of neutrophils (but also of monocytes), above all myeloperoxidase (MPO) and proteinase 3 (PR3). The other ANCA targets (BPI, elastase, cathepsin G, lactoferrin) are of no clinical relevance and should therefore not be sought.

ANCA research was based on the combination of two techniques: indirect immunofluorescence (IIF) on the one hand, and a technique studying the specificity of ANCA compared to MPO and PR3 on the other. The specific tests for the detection of anti-MPO and PR3 vary from center to center but are based above all on ELISA, flow fluorimetry (Luminex® instrument), or the dot blot. International consensus proposed in 2017 that tests targeted on the antigen, both more sensitive and specific than IIF, should be used as a first-line screening method for patients suspected of ANCA-associated vasculitides, e.g., without going through a preliminary screening on IIF

However, specific immunoassays can be used by default, with false negatives, and can be supplemented by the IIF or another second-line validation test in the event of a strong clinical suspicion

In severe forms, the rapid dot blot detection technique makes it possible to obtain an anti-PR3 and anti-MPO result in a few hours

The detection of ANCA with anti-PR3 or anti-MPO specificity may, in the absence of a histological confirmation, be sufficient in a suggestive clinical context for retaining the diagnosis of vasculitis combined with ANCA and rapidly starting the treatment