Factor XI Inhibitor Profile, Comprehensive

CPT: 85270; 85335; 85730; 85732(x3)
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Test Details


  • Bethesda Titer

Test Includes

Activated partial thromboplastin time (aPTT); aPTT 1:1 mix with normal plasma; aPTT 1:1 mix with saline; factor XI activity; factor XI Bethesda titer


Confirmation and characterization of factor XI 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 XI 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 XI activity, and are then incubated for two hours. An aPTT-based factor XI assay using factor XI-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 XI inhibitor that neutralized 0.5 IU of factor XI in this system. The number of serial dilutions tested is based on the anticipated level of the inhibitor.

Additional Information

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. 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. 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 anamnestic responses to therapy. Assays must also be performed repeatedly during inhibitor suppression therapy.

Specimen Requirements


Plasma, frozen


6 mL (2 mL in each of three tubes)


Blue-top (sodium citrate) tubes

Patient Preparation

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


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.

Storage Instructions

Freeze. Stable at room temperature for four hours.

Clinical Information


1. Adcock DM, Kressin DC, Marlar RA. Effect of 3.2% vs 3.8% sodium citrate concentration on routine coagulation testing. Am J Clin Pathol. 1997 Jan; 107(1):105-110. 8980376
2. Reneke J, Etzell J, Leslie S, Ng VL, Gottfried EL. Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant. Am J Clin Pathol. 1998 Jun; 109(6):754-757. 9620035
3. National Committee for Clinical Laboratory Standardization. Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline. 5th ed. Villanova, Pa: NCCLS; 2008. Document H21-A5:28(5).
4. Gottfried EL, Adachi MM. Prothrombin time and activated partial thromboplastin time can be performed on the first tube. Am J Clin Pathol. 1997 Jun; 107(6):681-683. 9169665
5. McGlasson DL, More L, Best HA, Norris WL, Doe RH, Ray H. Drawing specimens for coagulation testing: Is a second tube necessary? Clin Lab Sci. 1999 May-Jun; 12(3):137-139. 10539100
6. Adcock DM, Bethel MA, Macy PA. Coagulation Handbook. Aurora, Colo: Esoterix−Colorado Coagulation; 2006.


Goodnight SH, Hathaway WE, Disorders of Hemostasis and Thrombosis: A Clinical Guide. 2nd ed. New York, NY: McGraw-Hill; 2001.


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
500396 Factor XI Inhibitor 500513 APTT sec 14979-9
500396 Factor XI Inhibitor 500363 APTT 1:1 NP sec 5946-9
500396 Factor XI Inhibitor 500364 APTT 1:1 Saline Sec 43734-3
500396 Factor XI Inhibitor 500657 APTT 1:1 NP, 60 min Incubation sec 30323-0
500396 Factor XI Inhibitor 500658 APTT 1:1NP 60min Inc. Control sec 44950-4
500396 Factor XI Inhibitor 500674 Factor XI Activity % 3226-8
500396 Factor XI Inhibitor 500394 Factor XI Inactivator Assay Bethesda 3224-3

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