ANA fluorescence patterns and low C4 are associated with higher antiphospholipid antibody serological intensity in routine diagnostic testing

Abstract Objectives To evaluate whether antinuclear antibody (ANA) titre or HEp-2 fluorescence patterns are associated with antiphospholipid antibody (aPL) serological intensity, and to examine component-specific complement abnormalities – particularly low C4 – as laboratory correlates compatible with classical-pathway involvement. Methods This retrospective cross-sectional study analysed 626 consecutive diagnostic samples with same-day testing for lupus anticoagulant (LA), anticardiolipin (aCL), anti-β 2 -glycoprotein I (anti-β 2 GPI) antibodies, and ANA. aPL serological intensity was categorised from 0 to 3 (negative to triple-positive) using predefined laboratory cut-offs. ANA patterns were grouped as negative, homogeneous, speckled, or other. Complement variables included low C3, low C4, and combined hypocomplementaemia. Associations were assessed using ordered logistic regression, with Firth bias-reduced logistic regression applied for binary outcomes with sparse events. Results Overall, 26.5 % of samples (166/626) were aPL-positive, including 1.6 % triple-positive cases (10/626). High-titre ANA (≥1:160) was not associated with higher aPL serological intensity (OR 1.00, 95 % CI 0.51–1.96). In contrast, homogeneous (OR 1.99, 95 % CI 1.26–3.18) and speckled (OR 2.85, 95 % CI 1.66–4.91) ANA patterns were independently associated with higher aPL categories. Low C4, – but not low C3 – was associated with aPL positivity in binary analysis (Firth OR 6.84, p=0.001; low-C4 events n=10). Conclusions ANA fluorescence patterns, but not ANA titre, are associated with higher aPL serological intensity, indicating a distinct laboratory serological phenotype. Low C4 shows an association compatible with classical-pathway complement involvement. These findings are observational and require validation in longitudinal studies.