A two‐step coagulation test to identify antiβ2‐glycoprotein I lupus anticoagulants

Summary. Lupus anticoagulants (LA) are immunoglobulins which inhibit phospholipid (PL)‐dependent coagulation tests. LA are not specific, as they may reflect the presence of antibodies to human prothrombin, human β₂‐Glycoprotein I (β₂GPI), an association of previous antibodies or other antibodies. Antibodies to human β₂GPI act as in vitro anticoagulants by enhancing the binding of β₂GPI to PL, and this binding may be influenced by calcium ion concentration. A reduction in final calcium concentration, from 10 mm to 5 mm , increased coagulation times in both dilute Russell Viper Venom Time (dRVVT) and dilute Prothrombin Time (dPT) when plasmas of patients with antiβ₂GPI antibodies were used. Ten LA patients showed increased dRVVT and dPT ratios from means of 1.5 to 1.7 (P < 0.001) and 2.4 to 4.3 (P = 0.002), respectively. Instead, all LA‐positive antiβ₂GPI antibody‐negative patients showed decreased coagulation times from mean ratios of 1.5 to 1.3 (P = 0.004) in dRVVT and from 2.0 to 1.5 (P = 0.01) in dPT. These results are confirmed by running dRVVT of normal plasma spiked with affinity purified IgG antiβ₂GPI antibodies. Therefore, when a PL–dependent coagulation test is run twice, at different final calcium concentrations, antiβ₂GPI LA can be identified.