A factor V inhibitor: in vitro interference by calcium.

A patient with a factor V inhibitor resistant to all standard methods of therapy was recently reported. Because of this resistance, further investigation of the inhibitor was carried out employing the inhibitory plasma, serum, and isofocused fraction. It has subsequently been shown in vitro by use of the prothrombin time, partial thromboplastin time, and Russell's viper venom time that the activity of the inhibitor is diminished by the addition of calcium chloride or calcium gluconate to the inhibitory plasma, serum, or isofocused fraction before the addition of normal pooled plasma. This inhibitor, an IgG4 (λ) immunoglobulin appears to have a high calcium binding affinity and is markedly potentiated in acid pH. These findings suggest that there may be a role in vivo for calcium infusion as part of the clinical management of such an inhibitor and that the metabolic acidosis associated with hemorrhagic shock may potentiate this type of inhibitor.