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Heparin, Anti-Xa

Test code: 30292

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.1

The Heparin, Anti-Xa test is calibrated using unfractionated heparin (UFH) and low molecular weight heparin (LMWH) commercial standards. The commercial LMWH standard is traceable to the second international LMWH 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).2.3

However, it should be noted that any medication with anti-Xa activity, such as Arixtra® (fondaparinux sodium),  Xarelto® (rivaroxaban), Eliquis® (apixaban), or Savaysa® (edoxaban) can also interfere in this assay.  

The correct test to order is Fondaparinux Sodium (test code 16103). This test uses the factor Xa inhibition method, calibrated to fondaparinux sodium. Results are reported in mcg/mL units. Blood for fondaparinux levels should be drawn 3 hours after injection, after steady-state levels are reached, typically after the third or fourth once-daily dose.4

No, the correct test for measuring this oral factor Xa inhibitors are Rivaroxaban (test code 90981) and Apixaban (test code 94223). It is important to remember that anti-Xa inhibition assays are used but require calibration using the specific drug.

For LMWH monitoring, routine monitoring is not recommended. Though there is no therapeutic range for LMWH, observed peak anti-Xa level for q12h-hour dosing (eg, enoxaparin 1 mg/kg q12h) is 0.5 to 1 units/mL, and observed peak anti-Xa level for q24h-dosing of LMWHs (eg, enoxaparin 1.5 mg/kg q24h) is 1 to 2 units/mL. Peak levels should be obtained 4 hours after a dose, after steady-state levels are reached, which may occur at least after the third dose or after waiting 5 to 7 days.5,6

For UFH monitoring, since this medication is typically given inpatient, licensed providers should utilize local hospital tests and protocols for monitoring. While testing is typically performed 6 hours after a change in dosing, the licensed provider should be aware that there may be delays in sending such testing to Quest Diagnostics laboratories and should confirm that testing can be performed in a timely manner.  

References

  1. STA-Liquid Anti-Xa, 2014, DIAGNOSTICA STAGO
  2. Gilbert M, Goret N, Roland Y, et al. Validity of the new STA®-Calibrator LMWH for assaying most commonly used LMWH preparations with STA®-Rotachrom® heparin. Throm and Haemost. 2001;86(suppl 1).
  3. McGlasson DL. Using a single calibration curve with the anti-Xa chromogenic assay for monitoring heparin anticoagulation. Lab Med. 2005;36(5):297-299. doi:10.1309/HYCDUCQTV8YN145Y  
  4. Funk DM. Coagulation assays and anticoagulant monitoring. Hematology Am Soc Hematol Educ Program. 2012;2012:460-465. doi:10.1182/asheducation-2012.1.460
  5. Yentz S, Onwuemene OA, Stein BL, Cull EH, McMahon B. Clinical Use of Anti-Xa Monitoring in Malignancy-Associated Thrombosis. Thrombosis. 2015;2015:126975. doi:10.1155/2015/126975
  6. Duplaga BA, Rivers CW, Nutescu E. Dosing and monitoring of low-molecular-weight heparins in special populations. Pharmacotherapy. 2001;21(2):218-234. doi:10.1592/phco.21.2.218.34112

 

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 the physician’s education, clinical expertise, and assessment of the patient.

 

Document FAQS.53 Version: 2

Version 2 effective 10/23/2025 to present

Version 1 effective 03/10/2014 to 10/23/2025

Version 0 effective 04/20/2012 to 03/09/2014

 

 

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