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Heparin, Low Molecular Weight (Xa Inhibition) and Heparin, Unfractionated (Xa Inhibition)

Test code(s) 30292X, 404X

The FAQ information attached by the previously provided link has been retired. The link continues to be available for your historic reference. Current FAQs can be viewed at QuestDiagnostics.com/FAQs.

Version 0 Effective 4/20/12-3/10/14

Current version available here

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

These tests use 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 either an unfractionated heparin (UFH) or low molecular weight heparin (LMWH) specific calibration curve. Results are reported as anti-Xa IU/mL. Hence, the test is commonly referred to as an "anti-Xa" or "Xa Inhibition" assay.

Question 2. My patient is on Lovenox®. Is the test Heparin Anti-Xa (Low Molecular Weight Heparin) specific for Lovenox?

The Heparin Anti-Xa (Low Molecular Weight Heparin) test is calibrated using a commercial standard that is traceable to the low molecular weight heparins second international standard. Published studies have indicated that the commercial standard can be used to measure the more commonly used LMWHs: Dalteparin (Fragmin®), Enoxaparin (Lovenox®) and Tinzaparin (Innohep®).

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

Question 3. My patient is on unfractionated heparin (UFH). What test should I order to measure heparin concentration?

The Heparin, Anti-Xa, UFH test (test code 404X) should be ordered because it is specifically calibrated for UFH. The Heparin Anti-Xa (Low Molecular Weight Heparin) test (test code 30292X) is calibrated for LMWH and should be used for patients on LMWH therapy.

Question 4. My patient is receiving fondaparinux sodium (Arixtra®). 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/mLunits.

 

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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