Anti-Xa activity (UFH, LMWH, rivaroxaban & fondaparinux monitoring)

Description: 
Chromogenic anti-Xa assays are used to monitor the concentration of heparin based or direct FXa inhibitor anticoagulation present in a patients plasma. This is measured indirectly by adding excess antithrombin to the patients sample along with a known quantity of FXa. Any anticoagulant present in the sample will inactivate the FXa, and the residual FXa is then measured with a specific chromogenic substrate. Therefore the amount of colour change produced by the substrate is inversely proportional to the concentration of anticoagulant present. Drug specific calibrations are in use for unfractionated heparin (UFH), low molecular weight heparin (LWMH), fondaparinux, orgaran (danaparoid), rivaroxaban, apixaban and edoxaban. Therefore anticoagulant information is required at the point of request.

In the anti-Xa assay, the anticoagulant in the test plasma is reacted with fixed amounts of excess exogenous antithrombin and FXa. The anticoagulant forms a complex with the antithrombin and FXa and the residual FXa is reacted with a chromogenic substrate, the intensity of the coloured product being inversely proportional to the concentration of circulating anticoagulant.

Rivaroxaban is a direct FXa inhibitor and is assayed in a similar way except that no exogenous antithrombin is required.
Clinical details: 
LMWHs have more predictable bioavailability than UFH so only require 'monitoring' in certain clinical situations where dose calculation by body weight is unreliable. The APTT is normally sufficient for effective UFH monitoring, but it is relatively insensitive to the more targeted action of LMWH or fondaparinux and the anti-Xa assay is employed for this purpose. Apixaban, Rivaroxaban and Edoxaban are direct FXa inhibitor’s with predictable bioavailability and regular monitoring unnecessary. Measurement of levels be required in certain circumstances, such as suspected overdose, suspected non-compliance, renal failure and prior to surgery.
Related condition or disease: 
Therapeutic anticoagulation
Reference range: 

Therapeutic intravenous unfractionated heparin target anti-Xa activity = 0.5-1.0 iu/mL.
Prophylactic dalteparin: peak target <0.3 iu/mL & trough target <0.2 iu/mL
Therapeutic dalteparin administered once daily, peak target 0.8-1.2 iu/mL & trough target <0.3 iu/mL
Therapeutic dalteparin administered twice daily, peak target 0.5-1.0 iu/mL.
Prophylactic enoxaparin: peak target <0.3 iu/mL & trough target <0.2 iu/mL
Therapeutic enoxaparin administered once daily, peak target 0.8-1.2 iu/mL & trough target <0.3 iu/mL.
Tinzaparin therapeutic peak target anti-Xa activity is 0.5-1.0 iu/ml.
Fondaparinux therapeutic peak target anti-Xa activity is 0.8-1.2 µg/mL.
Danaparoid therapeutic peak target anti-Xa activity is 0.5-1.0 iu/ml.
Apixaban 2.5mg bd: peak levels should be <220ng/ml
Apixaban 5mg bd: peak levels should be <320ng/ml.
Rivaroxaban 10mg od: peak levels should be <195 ng/mL
Rivaroxaban 20mg od: peak levels should be <420ng/ml.
Edoxaban dose of 60mg od: peak levels should be <320 ng/mL

Units: 
IU/ml (UFH & LMWH) µg/mL (fondaparinux) ng/ml (apixaban, rivaroxaban and edoxaban)
Department: 
Haemostasis and Thrombosis Department
Location: 
St Thomas' Hospital

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Last updated: 09/07/2021