Factor VIII Inhibitor Profile, Comprehensive

Plasma, frozen

3 mL

Blue-top (sodium citrate) tube

The patient should not be anticoagulated. Do not draw from an arm with a heparin lock or heparinized catheter.

Confirmation and characterization of factor VIII inhibitor

If there is residual coagulation factor activity, it could falsely lower the Bethesda titer result. Lupus anticoagulant activity and antithrombotic agents that function as inhibitors should be ruled out prior to assaying for factor inhibitors in order to avoid factitiously positive results. Some inhibitors may fully neutralize factors in less than two hours. Autoimmune inhibitors that demonstrate second order kinetics cannot always be accurately measured in the Bethesda titer system.⁶

The factor VIII inhibitor (Bethesda titer) assay is performed using an activated partial thromboplastin time (aPTT)-based test system.⁶ Serial dilutions are made of patient plasma with veronal buffered saline, then mixed with normal plasma containing close to 100% factor VIII activity and are then incubated for one hour. An aPTT-based factor VIII assay using factor VIII-depleted plasma substrate is then performed on these incubated mixtures. Results are compared to those of incubated normal plasma. One Bethesda unit is defined as the amount of factor VIII inhibitor that neutralized 0.5 IU of factor VIII in this system. The number of serial dilutions tested is based on the anticipated level of the inhibitor.

Factor inhibitors (also called circulating anticoagulants or inactivators) are endogenously produced antibodies that interfere with coagulation in vivo or in vitro.⁶ These inhibitors are frequently specific for their respective coagulation factors. Specific factor inhibitors can be classified as neutralizing or non-neutralizing. Neutralizing inhibitors interact with the functional component of the coagulant protein while non-neutralizing antibodies react with the protein somewhere other than the functional epitopes. Factor inhibitors are not normally present in plasma. When present, factor inhibitors are measured by Bethesda titer units (BU). Titers <5 BUs are classified as low responders, titers >10 BUs as high responders. Factor VIII inhibitors may arise as IgG4 (noncomplement fixing) alloantibodies or autoantibodies. Factor VIII alloimmunization occurs in 15% to 20% of hemophilia, when the patient's plasma factor VIII concentration is <1%. Alloimmunization is idiosyncratic, affecting some, but not all, severe hemophilia A patients. Autoimmune factor VIII inhibitors occur in 1 of 1,000,000 individuals, primarily in pregnancy and the postpartum period, in patients older than 55 years of age, and in people with primary autoimmune disorders.

Porcine factor VIII inhibitors may arise in hemophilia A patients treated with porcine factor VIII (HyateC®) therapy. HyateC® is no longer available from the manufacturer. Alloimmune factor VIII inhibitors are suspected when hemophilia A patients' symptoms increase in severity, or bleeding continues despite infusion of ever-increasing dosages of factor concentrates. The normal plasma half-life of factor VIII, is 12 hours; however, hemophilia A patients with inhibitors may have undetectable factor VIII levels a few minutes after factor VIII-concentrate infusion and may require specialized therapy to halt acute bleeding. Neutralizing factor inhibitors form covalent complexes with their target coagulant factor molecules, rendering the factors nonfunctional. For alloimmune factor VIII inhibitor, this is a first order reaction with complete in vitro inactivation of available factor VIII. Autoimmune factor VIII inhibitors generally exhibit in vitro second order kinetics, achieving equilibrium with factor VIII leaving some residual factor VIII unbound in the plasma. In vivo, however, autoimmune factor VIII inhibitors cause “acquired hemophilia” with symptoms as severe as the more common congenital severe hemophilia. The mortality rate for acquired hemophilia is up to 22%.