Factor VII Inhibitor Profile, Comprehensive

Plasma, frozen

6 mL (2 mL in each of three tubes)

Blue-top (sodium citrate) tubes

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

Confirmation and characterization of factor VII 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 must be ruled out prior to assaying for factor inhibitors in order to avoid factitiously positive results. Autoimmune inhibitors that demonstrate second order kinetics cannot always be accurately measured in the Bethesda titer system.

The factor VII 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 VII activity and are then incubated for two hours. A PT-based factor VII assay using factor VII-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 VII inhibitor that neutralized 0.5 IU of factor VII 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 and may cause clinical bleeding. Non-neutralizing antibodies react with the protein somewhere other than the functional epitopes and may be clinically silent or may result in increased clearance of the factor. Factor inhibitors are not normally present in plasma. When present, factor inhibitors are measured by Bethesda titer units (BU). Titers <5 BUs are generally classified as low responders, titers >10 BUs as high responders. Responder status influences the approach for clinical treatment. Factor inhibitor assays are ordered in response to clinical impression, and are performed periodically on individuals who have inhibitors and are receiving therapy. During therapy, periodic assays are used, both to manage the dosage and to establish whether a patient is a low or high responder. Low responders maintain titers consistently <5 BUs, while high responders may generate extremely high titers and rapid amnestic responses to therapy. Assays must also be performed repeatedly during inhibitor suppression therapy.