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.
The patient should not be anticoagulated
Refrigerated - NO
Ambient - NO
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.
|aPTT||22.9 − 30.2 sec|
|aPTT 1:1 Normal plasma||22.9 − 30.2 sec|
|aPTT 1:1 Mix saline||Not established|
|aPTT 1:1 NP mix, 60 min incub||22.9 − 30.2 sec|
|Factor VIII Activity||57 − 163 %|