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Anti-Xa Result is Zero

From Jennifer Watson, Centracare Health:
Hello, What are your recommendations when an anti factor Xa returns with a value of 0, even after increasing doses several times? Thank you.

Hello, Jennifer, and thank you for this question. Assuming the heparin dosing has been administered correctly, you may be seeing a patient who has acquired, or even congenital, antithrombin deficiency. Most chromogenic anti-Xa heparin assay kits marketed in the US rely on patient antithrombin for an accurate measurement; these indirectly provide a presumptive indication of antithrombin deficiency by returning lower than expected results. Should you choose to back up the assay with a PTT, it is likely to be affected the same way, meaning its result would be shorter than expected. In this case, your physicians, to ensure the heparin therapy is protective, may choose to administer antithrombin concentrate. Thank you for your question, and I invite additional comments from our participants who may have encountered similar situations.

Comments (2)
Anticoagulant Therapy
Feb 10, 2016 11:05pm

is there hemolysis in the
Is there hemolysis in the sample? This could also depress the anti-Xa.

Feb 10, 2016 5:45pm

If antithrombin deficiency
If antithrombin deficiency was ruled out, including AT type II deficiency with mutations in the heparin-binding site (many AT assays showed normal AT level in this type of defect), another cause of unmeasurable anti-Xa could be heparin resistance due to cancer; it is believed that cancer heparanase could be responsible for fast heparin cleavage and inactivation, some references are below:

Vlodavsky I, et al. Heparanase, heparin and the coagulation system in cancer progression.
Krajewski KC et al. Achievement of therapeutic anti-Xa levels in a proven heparin-resistant patient through the use of nontraditional high-dose enoxaparin.

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