Antibody profiles in the mosaic of 'seronegative' APS syndrome

Clinical manifestations, as distinct from thrombotic and obstetric morbidity, were recently included in the update of classification criteria of the antiphospholipid syndrome (APS). However, the existence of several patients with clinical manifestations suggestive of APS, but negative for criteria antiphospholipid antibodies (aPLs) [anti-cardiolipin antibodies (aCL), anti-β2-glycoprotein I antibodies (aβ2-GPI), and lupus anticoagulant] may suggest an update of diagnostic criteria. In this study, we analysed the prevalence of six non-criteria aPLs in a large monocentric cohort of patients with seronegative APS (SN-APS), to investigate their possible diagnostic role. aCL IgA, aβ2-GPI IgA, and aβ2-GPI Domain 1 antibodies were detected by chemiluminescence, anti-phosphatidylserine/prothrombin (aPS/PT) IgG, anti-vimentin/cardiolipin (aVim/CL) IgG, and anti-carbamylated-β2-glycoprotein I (aCarb-β2-GPI) IgG by ELISA in sera from 144 SN-APS patients. In SN-APS patients, aCL IgA was detected in 4/144 (2.77%), aβ2-GPI IgA in 2/144 (1.39%), aβ2-GPI-Domain 1 in 1/144 (0.69%), aPS/PT in 16/144 (11.11%), aVim/CL in 37/144 (25.69%), and aCarb-β2-GPI in 43/144 patients (29.86%). Patients negative for all non-criteria aPL assays were 77/144 (53.47%). Notably, the Venn diagram showed that aCarb-β2-GPI together with aVim/CL represented the prevalent combination of positive antibodies. In SN-APS patients, aCL IgA were associated with recurrent thrombosis (OR 11.48; P = 0.03); in obstetric SN-APS patients, aPS/PT were significantly associated with foetal deaths (OR 4.84; P = 0.01), aVim/CL with spontaneous abortions (OR 2.71; P = 0.016). This study indicates that aPS/PT, aVim/CL and aCarb-β2-GPI antibodies may represent useful tools to identify 'seronegative' APS patients, who are negative for criteria aPLs, supporting the need to make testing for non-criteria aPLs more accessible in patients with SN-APS.