Diagnosis: Prekallikrein deficiency
• The contact system consists of factor XII, factor XI, and prekallikrein (serine protease zymogens) and the nonenzymatic cofactor high-molecular-weight kininogen.
• Deficiencies in the contact system proteins can result in an elevated activated partial thromboplastin time (aPTT) with a normal prothrombin time (PT). A mixing study with pooled normal plasma would correct the aPTT (i.e., deficiency, not an inhibitor).
• Factor XII, prekallikrein, and high-molecular-weight kininogen deficiencies are not associated with bleeding.
• Complete normalization of prolonged aPTT following prolonged incubation is characteristic of prekallikrein deficiency and is caused by autoactivation of factor XII (activation of factor XII by factor XIIa).
• Although autoactivation of factor XII has been described with lupus anticoagulants (LACs), normalization of the aPTT into the reference range does not occur. In addition, LACs are not associated with correction after mixing with pooled normal plasma.