BackgroundAntineutrophil cytoplasmic antibodies (ANCA) are the serological marker of some idiopathic systemic vasculitides, predominantly involving small and medium-sized blood vessels, such as granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis (EGPA), which are known as the ANCA-associated vasculitis (AAV). Nevertheless, ANCA have been reported in a number of other conditions.ObjectivesTo retrospectively evaluate ANCA diagnostic accuracy in a cohort of unselected patients.MethodsFrom January 2014 to December 2016 a total of 6781 serum samples with a test request for ANCA were submitted to the Immunology Department of a 1.000-bed tertiary teaching hospital from Barcelona (Spain), from both inpatients and outpatients.Indirect immunofluorescence (IIF) was performed for all requests using a commercially available “Granulocyte Mosaic 13” (EUROIMMUN). IIF allowed recognition of three staining patterns: cytoplasmic (cANCA), perinuclear (pANCA) and atypical (xANCA). For the detection of antibodies against mieloperoxidase (MPO) and proteinase 3 (PR3) a chemiluminescent immuno-assay (CLIA) using commercially available ”QUANTA Flash MPO/PR3” (INOVA diagnostics) was performed in patients with positive IIF.We reviewed the clinical charts of patients that underwent ANCA testing and collected patients’ diagnoses, as established by their treating physician one year after sampling. In the event of multiple ANCA test in a single patient we include only the first test request (we excluded 1323 tests performed in 661 patients). We also excluded 184 patients with insufficient information and 306 ANCA tests with no diagnostic purpose. Therefore the study population includes 4968 patients.Statistical analysis was performed with Stata 14.2 (College Station, TX, USA). Diagnostic performance was assessed using sensitivity, specificity, positive likelihood ratio (LR+), positive and negative predictive values (PPV and NPV) and global efficiency. Confidence Intervals (CI) were calculated using Wilson method.ResultsOnly 34 patients (0.68%) received a diagnosis of AAV: 25 MPA, 6 GPA and 3 EGPA.Sensitivity[CI 95%]Specificity[CI 95%]LR+[CI 95%]PPV[CI 95%]NPV[CI 95%]Efficiency[CI 95%] Positive IIF94.1%87.6%7.65%99.95%87.7%[80.9–98.4][86.7–88.5][6.8–8.5][3.5–6.9][99.83–99.99][86.7–88.5]IIF Typical pattern91.2%96.9%2916.7%99.94%96.8%[77–97[96.3–97.3][24.1–35][12–22.7][99.82–99.98][96.3–97.3]cANCA/PR3 or pANCA/MPO76.5%99.1%83.836.6%99.84%98.9%[60–87.6][98.8–99.3][59.3–118.4][26.4–48.2][99.68–99.92][98.6–99.2]The majority (87.1%) of patients had a negative ANCA test and only 12.9% were found positive by IIF. Among 643 positive patients IIF pattern distribution was: 457 (71.1%) atypical, 108 (16.8%) perinuclear and 78 (12.1%) cytoplasmic pattern. Among patients with positive ANCA 32 (5%) had an AAV. Two patients with AAV had negative ANCA (one GPA and one EGPA).Data of diagnostic accuracy of ANCA for AAV are showed in table 1:ConclusionsANCA testing with commonly commercially availa...