Analytical and diagnostic accuracy of the EliA automated enzyme fluoroimmunoassay for antineutrophil cytoplasmic autoantibody detection.

Anti-proteinase 3 antineutrophil cytoplasmic antibodies (PR3-ANCA) and anti-myeloperoxidase antibodies (MPO-ANCA) are considered important serological markers for several forms of idiopathic systemic vasculitis. The aim of the study was to verify the analytical and clinical performance of a new automated enzyme fluoroimmunoassay, the EliA system, for PR3-ANCA and MPO-ANCA detection. For this purpose the sera of 52 consecutive well-defined patients with a clinical diagnosis of Wegener's granulomatosis (WG) (n=29) or microscopic polyangiitis (MPA) (n=23), and 70 controls suffering from connective tissue disease (25 systemic lupus erythematosus, 25 Sjögren's syndrome and 20 rheumatoid arthritis) were tested for PR3-ANCA and MPO-ANCA with the EliA assay (Pharmacia Diagnostics, Freiburg, Germany). For comparison purposes, the same sera were also tested by indirect immunofluorescence, another direct immunometric assay (Varelisa, Pharmacia Diagnostics) and a capture PR3-ANCA (Wieslab AB, Lund, Sweden) method. Both the EliA PR3-ANCA and MPO-ANCA assays showed between- and within-assay precision of <10%. The dilution test gave straight lines (r2=0.998) for both antibody assays. The recovery ranged from 97.9% to 102.7% for PR3-ANCA and from 84.9% to 91.4% for MPO-ANCA. There was a high positive correlation between the EliA and Varelisa methods for quantitative detection of MPO-ANCA levels (r2=0.949) and a lower correlation for PR3-ANCA (r2=0.771). Conversely, poor correlation was observed between EliA PR3-ANCA and capture PR3-ANCA (r2=0.537). The overall sensitivity and specificity of EliA PR3-ANCA and MPO-ANCA for the vasculitides considered in this study were 82.7% and 97.2%, respectively, with a positive predictive value of 96.6% and a negative predictive value of 84.9%. Comparison of the results obtained with the indirect immunofluorescence, Varelisa and capture PR3-ANCA methods showed that the indirect immunofluorescence assay is the most sensitive method for the diagnosis of vasculitis (88.5%), but the least specific (94.3%); the EliA method is slightly more specific (97.2%) than the Varelisa method (95.7%), and also slightly more sensitive (82.7% vs. 80.8%). Capture PR3-ANCA proved to be the most sensitive method for detection of anti-proteinase 3 antibodies in WG (89.7% vs. 86.2% EliA and 79.3% Varelisa). In conclusion, the EliA MPO-ANCA and PR3-ANCA methods provide good diagnostic accuracy and excellent analytical accuracy, which, in association with the practicality of the automated EliA system, make this method a useful tool for the diagnosis of ANCA-associated vasculitides.