Anti factor Xa

Keywords: Anti Xa, Heparin assay

Specimen:

4.5 mL blood added to 0.5 mL citrate.

When requesting assay, information on current anticoagulant use needs to be specified.

Method:
  1. Chromogenic assay; residual factor Xa measured after neutralisation with AT III. Chromogenic substrate is activated by residual factor Xa.
  2. Coagulation-based assay.
Therapeutic Interval:

Monitoring is not required for prophylactic dosing.

If monitoring is required for therapeutic dosing, the therapeutic range will differ according to mode and frequency of administration and according to anticoagulant.

Consult pathologist.

Application:

Monitoring of (full dose) low molecular weight heparin (LMWH) therapy is not generally required, except in renal failure, extremes of body weight, pregnancy or other situations where there is an increased risk of bleeding. LMWH should be used with care and monitoring in patients with any abnormality of renal function, particularly the elderly. Monitoring should be done in consultation with a haematologist.

Monitoring of routine LMWH prophylaxis is not cost effective, is not required to achieve clinical efficacy and is not indicated to predict risk of bleeding, which is minimal with prophylactic doses in patients with normal renal function.

In consultation with a haematologist or pathologist, may also be used in exceptional circumstances for monitoring of unfractionated heparin, fondaparinux and oral direct Xa inhibitor (rivaroxaban, apixaban) therapy. Therapeutic ranges vary according to the anticoagulant– consult pathologist.

Interpretation:

Most patients on low molecular weight heparin, pentasaccharide (fondaparinux) and oral direct Xa inhibitor (rivaroxaban, apixaban) therapy do not require monitoring.

Reference:

Walenga J et al. Thromb Res 1991; 14: 49-62.

Hirsch J, Levine MN. Blood 1992; 79: 1-17.

Garcia DA1, Baglin TP, Weitz JI, Samama MM; American College of Chest Physicians. Parenteral anticoagulants: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012 Feb;141(2 Suppl):e24S-43S