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Heparin, Anti-Xa

Test code(s) 30292

Question 1. What method is used to measure heparin levels?

This test uses the factor Xa inhibition method. The heparin-antithrombin III complex present in patient plasma binds and inhibits factor Xa, which is added in excess during testing. Residual factor Xa enzymatic activity is measured using a chromogenic substrate. The rate of substrate hydrolysis is then compared to a heparin calibration curve. Results are reported in anti-Xa IU/mL units. Hence, the test is commonly referred to as an "anti-Xa" or "Xa Inhibition" assay.

This test is calibrated using unfractionated heparin (UFH) and low molecular weight heparin (LMWH) standards. Thus, it is suitable for monitoring both UFH and LMWH therapy.

Question 2. My patient is on Lovenox®(enoxaparin). Is this test specific for Lovenox?

The Heparin, Anti-Xa test is calibrated using UFH and LMWH commercial standards. The commercial LMWH standard is traceable to the low molecular weight heparin second international standard. Published studies have indicated that the commercial standard can be used to measure the more commonly used LMWHs: Fragmin®(dalteparin), Lovenox®(enoxaparin), and Innohep®(tinzaparin).

Reference: Abstract #CD3224. Supplement to the Journal Thrombosis and Haemostasis, July 2001 (ISSN 0340-6245)

Question 3. My patient is receiving Arixtra®(fondaparinux sodium). What is the appropriate test to order?

The correct test to order is Fondaparinux Sodium (test code 16103X). This test uses the factor Xa inhibition method, calibrated to fondaparinux sodium. Results are reported in mcg/mL units.

Question 4. Can this assay be used to measure Xa inhibition induced by Xarelto®(rivaroxaban)?

No, the correct test for measuring this oral factor Xa inhibitor is Rivaroxaban (test code 90981).

 

Retired version 0 Effective 4/20/12-3/10/14

This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
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