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Plasma anti Xa assays may be helpful to monitor anticoagulation levels in patients receiving low molecular weight heparin (LWMH).  Trough levels are helpful to identify dose accumulation e.g. in renal impairment.  Peak levels are helpful to confirm adequate dosing e.g. in gross obesity.

Occasionally anti Xa assays are used to monitor patients receiving unfractionated heparin (UFH) - if they have a prolonged APTT prior to starting UFH or very high FVIII levels.  

At UHNM, the anti Xa assay is not calibrated to monitor patients on DOACs affecting anti Xa activity (Apixaban, Edoxaban, Rivaroxaban).  DOAC levels are a send away test.  In the emergency setting, if standard anti Xa levels are ≤0.1 for Apixaban, Edoxaban, rivaroxaban; then there is likely no significant anticoagulant effect and a reversal agent is not indicated.

The requesting and interpreting of anti Xa results should be guided by clinical haematology.

Indications for factor Anti Xa assay may include:

• Monitoring of LWMH
• Monitoring of UFH, danaparoid, Argatroban, Fondaparinux
• Emergency surgery/bleeding on anti Xa DOAC (Apixaban, Edoxaban, Rivaroxaban)

Other Comments:

Timing of sample: 

• Timing of anti Xa levels depends on the indication for the test
• Trough levels -  Take just prior to LWMH dose
• Peak levels -  Take 3 hours after LWMH dose

Result interpretation: 

• Results should be discussed with clinical haematology taking into account the type of anticoagulation, anticoagulation dosing frequency, clinical situation and timing of the test 
• In patients receiving an anti Xa DOAC (apixaban, edoxaban, rivaroxaban), if anti Xa level is ≤0.1 then likely no significant anticoagulant effect.  Reversal agent not indicated

An anti Xa level of 0.0 u/ml indicates no anticoagulation is detectable