Relationship between the formation of nicked β₂GPI and α₂PI activity in plasma. After 16-hour incubation of samples with 120 IU of UK at 37°C, the intact and nicked form of β₂GPI were separated as described in Materials and Methods. Horizontal axis and vertical axis correspond to α₂PI activity and amount of nicked β₂GPI (percentage of total β₂GPI) in the plasma. α₂PI activity was expressed as the percentage of normal plasma.