Borderline positive antineutrophil cytoplasmic antibodies (ANCA)-PR3/MPO detection in a large cohort tertiary center: lessons learnt from a real-life experience.

BACKGROUND: Enzyme-linked immunosorbent assay (ELISA) and indirect immunofluorescence (IIF) are the best strategies for antineutrophil cytoplasmic antibodies (ANCA) detection. In a minority of subjects, ELISA-based ANCA testing may result in a borderline positive titre. Therefore, we assessed the clinical significance of such a result.

METHODS: This is a retrospective study, which included all subjects screened for ANCA subtypes (myeloperoxidase (MPO) or proteinase-3 (PR3)) with subsequent identification of borderline positive results, as determined by ELISA and retested using IIF. The demographic, clinical and laboratory data of subjects with borderline positive ANCA test results were extracted from their medical records.

RESULTS: A total of 14,555 PR3/MPO-ANCA tests were performed with ELISA during the study period (2006-2016). Of the 14,555 PR3-ANCA antibody tests that were performed, 94 were borderline positive (titre 0.9-1.1), and of 14,555 MPO-ANCA antibody tests, 43 were borderline positive (titre 0.9-1.1). The male-to-female ratio was 1:1.08 and the mean age was 50.95±21.79 years. Four MPO-ANCA (9.30%) and 11 PR3-ANCA (11.70%) antibody borderline samples resulted positive on IIF testing. Subjects with borderline positive MPO-ANCA were found to have a poorer outcome in terms of renal failure and the requirement of dialysis.

CONCLUSIONS: Subjects with borderline positive MPO-ANCA and positive p-ANCA (IIF) seem to have a less favorable outcome. Physicians should be aware of these findings and possibly perform a closer follow-up and routine screening for these subjects.