Confirmation and characterization of factor V inhibitor
The patient should not be on anticoagulant therapy
Frozen (preferred) – 30 days
Ambient – 4 hours
Refrigerated – NO
The factor V inhibitor (Bethesda titer) assay is performed using a prothrombin time-based system, using thromboplastin reagent.6 Serial dilutions are made of patient plasma with veronal buffered saline, then mixed with normal plasma containing close to 100% factor V activity, and are then incubated for two hours. A PT-based factor V assay using factor V-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 V inhibitor that neutralized 0.5 IU of factor V in this system. The number of serial dilutions tested is based on the anticipated level of the inhibitor.
Prothrombin and APTT, sec
Factor V activity, % and Facator V inact assay, <0.8 Bethesda