Harmonization of anti-nuclear antibody testing (ANA) by indirect immunofluorescence assay: Results from ten years of UK NEQAS external quality assessment

External quality assurance (EQA) programs play a pivotal role in monitoring laboratory practices, allowing each laboratory to evaluate the consistency of results across different methods as well the ability of individual laboratories to compare and improve over time their own performance. The objective of our study was to analyze the UK NEQAS EQA reports for the “Antibodies to Nuclear and Related Antigens” program from 2013 to 2023, to assess the overall level of harmonization of the responses for anti-nuclear antibody (ANA) testing by indirect immunofluorescence (IIF), in terms of both pattern and titer consensus. As a second aim, we analyzed the impact of the introduction in UK NEQAS EQA reports of the International Consensus on ANA Patterns (ICAP) nomenclature and of digital image recognition on the harmonization of the ANA HEp-2 IIF test. The percentage of consensus for positive/negative results was significantly higher (90.9 ± 1.4) in 2023 than in 2013 (64.0 ± 7.8, p < 0.001). Common to all years in the investigated period, consensus on pattern recognition was significantly lower for the homogenous pattern (70.5 ± 16.0) compared to the centromere (84.9 ± 14.9), the speckled (90.3 ± 12.3), and the negative (84.5 ± 18.6, p < 0.001) samples, while it was significantly higher for titers 1:80–1:320 than for titers > 1:320 (p < 0.001). The difference between manual reading and digital reading was not significant (93.8 % vs. 92.4 %; p = 0.078), but it was significant between the pre- and post-use of the ICAP nomenclature (82.6 % vs. 93.8 %; p < 0.001). This study shows that the variability in ANA recognition and reporting is pattern (homogeneous > speckled > centromere) and titer (high titer > low titer) dependent. While we did not find any difference between the use of manual reading compared to digital reading, the adoption of the ICAP nomenclature greatly improved the harmonization of ANA reporting.