• β-2 glycoprotein I;
  • calcium;
  • lupus anticoagulant;
  • thrombosis

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.