Performance evaluation of three assays for the detection of PR3-ANCA in granulomatosis with polyangiitis in daily practice

Background: Anti-neutrophil cytoplasmic antibodies (ANCA) directed against proteinase 3 (PR3-ANCA) are a serological hallmark of small vessel vasculitis, particularly granulomatosis with polyangiitis (GPA). To increase their sensitivity, some ELISA employ the human native PR3 combined with a recombinant protein. Their specificity in daily practice is still to be defined. Our objective was to compare the performance for GPA diagnosis of three PR3-ANCA assays in daily practice.

Patients and methods: Seventy-eight consecutive patients' sera with suggestive IIF were included. All sera were tested with a routine Enzyme Linked Immuno adsorbant Assay (ELISA) employing a mixture of human native and human recombinant (hn+hr) PR3 (EUROIMMUN™) compared to two assays using immobilized purified human PR3 (QUANTA Lite(®) ELISA and QUANTA Flash(®) Chemiluminescence assay (CIA), INOVA Diagnostics). Clinical data including BVAS score were collected retrospectively.

Results: Nineteen out of the 78 patients had GPA. The hn+hr PR3 ELISA had a good sensitivity (100%) but a lower specificity for the diagnosis of GPA (61.0%) than the assays using the sole native protein (hn ELISA: 81.4%, hn CIA: 69.5%). False positive results mainly consisted of patients with inflammatory bowel disease, who had a specific PR3-ANCA positivity assembly when coupling the assays. The antibody titers by human native PR3 assays, but not hn+hr assay, positively correlated with BVAS score.

Conclusion: These results highlight the need of a close collaboration between physicians and immunologists. Combining assays including last generation CIA employing human native antigens should improve the performance of GPA's diagnosis.