Anti-neutrophil cytoplasmic antibodies (ANCA) with a C-ANCA or P-ANCA pattern are detected in ANCA-associated vasculitis (AAV). While in most AAV patients a C-ANCA pattern is due to reactivity with proteinase-3 (PR3)-ANCA, some C-ANCA positive sera do not react with PR3.
Objective: The development and evaluation of a direct ELISA for PR3-ANCA with increased sensitivity.
Methods: A mixture of human native (hn) and human recombinant (hr) PR3 was used as antigen coating. The resulting ELISA (Anti-PR3-hn-hr) was compared with ELISAs using directly coated hn-PR3 or hr-PR3, as well as with a hn-PR3 capture ELISA. Assay characteristics were determined in AAV patients (n=248), with special attention for those patients with C-ANCA (n=132), as well as disease controls (n=585) and healthy controls (n=429). Additionally, for prediction of relapses serial samples of 46 PR3-AAV patients were analysed.
Results: At a predefined specificity of 99% both ELISAs containing hr-PR3 revealed a substantial increase from 14.4 to 18.2% in sensitivity. For prediction of relapses by rises in PR3-ANCA titers the capture ELISA was most optimal (odds ratio 12.5). With an odds ratio of 8.9 the novel Anti-PR3-hn-hr ELISA was second best.
Conclusion: Due to the very high sensitivity of the novel Anti-PR3-hn-hr ELISA for detection of PR3-ANCA in C-ANCA positive samples of AAV patients this assay has an excellent diagnostic performance. This feature is combined with a good predictability of clinical relapses in PR3-AAV patients. These characteristics challenge the dogma that for detection of PR3-ANCA capture ELISAs are superior for diagnosis and follow-up.