Anti Xa (Anti factor Xa; Heparin assay) - plasma

Last updated: Monday, 06, August, 2007

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Item Process

4.5 mL blood added to 0.5 mL citrate.

  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, ranges depend on whether the subcutaneous or intravenous route is employed.

Continuous IVI infusion: 0.5-1.0  anti-Xa units/mL
Subcutaneous injection
trough: > 0.3 anti-Xa units/mL  
peak: < 1.0 anti-Xa units/mL.

Consult pathologist.


Monitoring of (full dose) low molecular weight heparin (LMWH) therapy is rarely required, except in renal failure, extremes of body weight or other situations where there is an increased risk of bleeding.

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.

LMWH should be used with care and monitoring in patients with any abnormality of renal function, particularly the elderly.


Most patients on low molecular weight heparin do not require monitoring.


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

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