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Factor IX Inhibitor Profile, Comprehensive
- Bethesda Titer
Activated partial thromboplastin time (aPTT); aPTT 1:1 mix with normal plasma; aPTT 1:1 mix with normal plasma (60-minute control); aPTT 1:1 mix with saline; factor IX activity; factor IX Bethesda titer
Confirmation and characterization of factor IX 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 be accurately measured in the Bethesda titer system.6
The factor IX inhibitor (Bethesda titer) assay is performed using an activated partial thromboplastin time (aPTT)-based system.6 Serial dilutions are made of patient plasma with veronal buffered saline, then mixed with normal plasma containing close to 100% factor IX activity and are then incubated for two hours. An aPTT-based factor IX assay using factor IX-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 IX inhibitor that neutralized 0.5 IU of factor IX 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 IX inhibitors arise as alloantibodies in 3% to 5% of hemophilia B (Christmas disease) patients in response to factor IX concentrate, prothrombin complex concentrate, or fresh frozen plasma therapy.
6 mL (2 mL in each of three tubes)
Citrated plasma samples should be collected by double centrifugation. Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood to anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples, except when using a winged blood collection device (ie, "butterfly"), in which case a discard tube should be used.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternate anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes. Centrifuge for 10 minutes and carefully remove 2/3 of the plasma using a plastic transfer pipette, being careful not to disturb the cells. Deliver to a plastic transport tube, cap, and recentrifuge for 10 minutes. Use a second plastic pipette to remove the plasma, staying clear of the platelets at the bottom of the tube. Transfer the plasma into a LabCorp PP transpak frozen purple tube with screw cap (LabCorp N° 49482). Freeze immediately and maintain frozen until tested. To avoid delays in turnaround time when requesting multiple tests on frozen samples, please submit separate frozen specimens for each test requested.
Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.
Freeze. Stable at room temperature for four hours.