Diagnostic Impact of ANA Interference on p-ANCA Interpretation in Autoimmune Testing
Abstract
Objective: Antineutrophil cytoplasmic antibodies (ANCA) are crucial for diagnosing ANCA-associated vasculitis (AAV). The indirect immunofluorescence assay (IIFA) is commonly used but may produce perinuclear ANCA (p-ANCA) positivity in non-AAV, especially in antinuclear antibody (ANA)-positive cases. Specific ANA patterns and high titers may increase the frequency of non-AAV–p-ANCA positivity. This study aims to assess the correlation and impact of ANA patterns and titers on p-ANCA interpretation by IIFA.
Material and Methods: This retrospective study analyzed ANA-positive serum samples collected between 2019 and 2024. IIFA tested ANA and ANCA, while antibodies to myeloperoxidase (MPO) and proteinase 3 (PR3) were assessed by enzyme-linked immunosorbent assay (ELISA). Only ELISA-negative cases were included. IIFA classified samples into non-MPO–p-ANCA-positive and p-ANCA-negative groups. ANA patterns and titers were interpreted using International Consensus on ANA Patterns (ICAP) guidelines. Chi-square and logistic regression identified factors associated with non-MPO–p-ANCA positivity.
Results: Non-MPO–p-ANCA positivity was identified in 23.9% of cases (n=32). The homogeneous ANA pattern showed a significant association (adjusted OR: 4.11, 95% confidence interval [CI]: 1.18–14.37, p-value=0.027). High ANA titer (≥1:1280) was also independently related to this outcome (adjusted OR: 4.47, 95% CI: 1.50–13.33, p-value=0.007), whereas intermediate titers were associated with a lower likelihood of positivity.
Conclusion: Homogeneous pattern and high titer were associated with non-MPO–p-ANCA positivity. Standardized ANCA testing, including ANA assessment and confirmatory MPO/PR3-ANCA ELISA, improves diagnostic accuracy and reduces misinterpretation in autoimmune serology.
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