Factor X Chromogenic Assay

Patients who have lupus anticoagulants associated with recurrent venous thrombosis are frequently placed on long-term oral anticoagulant therapy, however there is considerable controversy regarding the intensity of anticoagulant therapy these patients should receive. Some authorities recommend that the INR in these patients should be maintained at 3.0 or greater, while others maintain that the standard 2.0-3.0 range is adequate to prevent recurrent thrombosis.

One issue contributing to this problem is recent evidence indicating that lupus anticoagulants may cause variable prolongation of the PT, resulting in falsely elevated INR’s that do not accurately reflect the true level of anticoagulation. A recent study of patients with lupus anticoagulants who were receiving warfarin showed that INR’s obtained using different thromboplastins varied significantly, and often overestimated the extent of anticoagulation (Ann Intern Med, 127: 177-185, 1997). This interference with the INR may at least partially explain the difficulties that have been encountered in adequately anticoagulating these patients.

A chromogenic assay for factor X (one of the vitamin K dependent factors) has recently been recommended as a means of evaluating the true level of anticoagulation in patients with lupus anticoagulants. This is an assay for factor X activity, performed by a non-coagulation based method that reflects the extent of warfarin anticoagulation, but is not affected by a lupus anticoagulant. A factor X range of 38% to 23% is approximately equivalent to an INR range of 2.0 - 3.0 (note that the factor X level decreases as the INR increases). In some patients with lupus anticoagulants, it may be anticipated that the INR will appear to be in the therapeutic range (2.0-3.0 for example), while the factor X level will be inappropriately high (greater than 38%), indicating that the INR is falsely elevated by the lupus anticoagulant, while the patient is, in fact, inadequately anticoagulated.

This assay should be used only for patients who have established lupus anticoagulants. It should serve only as an intermittent guide to evaluate the level of anticoagulation, and should not be used routinely to monitor warfarin in these patients. It is suggested that the assay be utilized in the following manner:

  • When oral anticoagulation is started in a patient with an established lupus anticoagulant, the INR should be used to monitor the patient until the desired therapeutic range is achieved.
  • At this point, the factor X / INR assay should be ordered to check whether the factor X level is in the appropriate range (23-38%). If the factor X level is persistently high, the warfarin dose should be adjusted, aiming for a higher INR. When the factor X is in the appropriate range, note the corresponding INR.
  • The higher INR should then be used as the desired therapeutic level for continued monitoring of the patient.

A 5mL blue top tube is required.

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