RESUMO
OBJECTIVE: The objective of this study was to extend the findings of the preliminary study by measuring the avidity of IgG anti-ß2-glycoprotein I antibodies (anti-ß2-GPI) on a larger group of patients with primary or secondary antiphospholipid syndrome (APS) and anti-ß2-GPI positive patients without APS in the frame of the European Forum on antiphospholipid antibodies (aPL). METHODS: Serum from 137 patients with primary APS, APS associated with autoimmune diseases, and patients with autoimmune diseases other than APS from five EU rheumatology centres were tested for anti-ß2-GPI antibodies. The 109 patients who were sera positive for anti-ß2-GPI by the in-house anti-ß2-GPI enzyme-linked immunosorbent assay (ELISA) at the Immunology Laboratory, UMC Ljubljana were selected for further testing on avidity with chaotropic anti-ß2-GPI ELISA. RESULTS: High, low and heterogeneous avidity IgG anti-ß2-GPI was found in 32/109, 17/109 and 60/109 patients respectively. Significantly more patients with APS were in the high avidity than in the low avidity anti-ß2-GPI group, while the opposite was observed for non-APS (both p < 0.001). The most common clinical feature among patients with high avidity anti-ß2-GPI was thrombosis, mainly due to venous thrombosis (p < 0.01 and p < 0.001, versus low avidity anti-ß2-GPI group). CONCLUSION: Patients with or without APS had anti-ß2-GPI of high, low or heterogeneous avidity. High avidity anti-ß2-GPI was associated with thrombosis and APS, while in the low avidity anti-ß2-GPI group non-APS (predominantly SLE) patients prevailed. Determination of anti-ß2-GPI avidity should be considered in the analytical strategies for further differentiation of patients with anti-ß2-GPI antibodies.