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Anti-Factor Xa Assay

February 15th, 2021


  • Important Lab Points

  • Measures the level of anti-Xa activity in a sample.
  • Performed by adding a known amount of factor Xa to a plasma sample and then measuring the residual Xa level; this is inversely proportional to the amount of anti-Xa activity.
  • The result is measured against a curve specific to the medication being monitored - e.g. low molecular weight heparin, rivaroxaban, dabigatran, fondaparinux or danaparoid.
  • Practical Points

  • The factor Xa level may be used to monitor the efficacy of medications that inhibit factor Xa, such as low molecular weight heparins, rivaroxaban, apixaban, fondaparinux and danaparoid.
  • Monitoring of low molecular weight heparins (enoxaparin / dalteparin) using factor Xa is not routine and is reserved for specific circumstances such as renal impairment, pregnancy or obesity.
  • Monitoring of factor Xa inhibitors (rivaroxaban / apixaban), fondaparinux and danaparoid is generally not performed except in certain circumstances as guided by a haematologist.
  • The target range depends on the medication, dose and frequency.
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