Platelet factor V supports hemostasis in a patient with an acquired factor V inhibitor, as shown by prothrombinase and tenase assays.

A woman with gross hematuria was shown to have a severe isolated factor V deficiency due to a factor V inhibitor of 200 U/ml titer. Hematuria persisted despite multiple infusions of plasma but, after one transfusion with 1 U platelets, urine red blood cells decreased by more than 98%. To evaluate the patient's platelet function we performed prothrombinase and tenase assays with platelets from the patient and from normal donors. By prothrombinase assay, ionophore-activated patient platelets showed 42% of the activity of normal platelets in their ability to support prothrombin activation by activated factor X; whereas in a 'tenase' assay, which measures the platelets' ability to support factor X activation by activated factor IX + activated factor VIII, their activity was 117% of normal. The addition of excess bovine activated factor V to the prothrombinase assay fully corrected the defect. The results demonstrate the benefit of platelet transfusion and indicate that in this case the platelets are the primary source of factor V for hemostasis.