Anti factor-Xa; LMWH monitoring
Haematology
To determine the level of factor Xa inhibition.
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.
Monitoring of LMWH in not routinely required.
Occasionally anti Xa assays are used to monitor patients receiving unfractionated heparin (UFH) e.g. if they have a prolonged APTT prior to starting UFH or very high FVIII levels.
The anti Xa assay is not calibrated to monitor patients on DOACs affecting anti Xa activity (apixaban, edoxaban, rivaroxaban), however, in the emergency setting, anti-Xa levels may be helpful to exclude DOAC anticoagulant effect e.g. prior to urgent high-risk surgery. Monitoring of DOAC levels is not routinely required. Please see Apixaban Level for patients taking apixaban. All other DOAC levels are send away tests. Discuss with Clinical Haematology prior.
Indications for factor Anti Xa assay may include:
Requests must be approved by Clinical Haematology.
Urgent requests must be discussed with the laboratory.
Venous whole blood is preferred. For other sample types please discuss with the laboratory.
Timing of sample:
Packaging must be compliant with Dangerous Good Safety Advisor (DGSA) requirements click here for more information.
Samples should be delivered to the laboratory within 2 hours of venepuncture.
1 X Citrate (Blue)
Must be filled to minimum fill line.
Urgent – 4 hours (Must be discussed with laboratory)
Routine – 72 hours (Excluding Bank Holidays)
Interpretation is dependent on clinical situation, anticoagulation dose and timing of the test.
Results should be discussed with Clinical Haematology.
Anti Xa level of 0.0 iU/ml indicates no anticoagulation is detectable.
In the emergency setting, if standard anti Xa levels are ≤0.1iU/ml for apixaban, edoxaban, rivaroxaban; then there is likely no significant anticoagulant effect and a reversal agent is not indicated.
Critical Values / Critical Result Communication
Results from urgent requests will be phoned to the requesting source.
Factors that significantly affect result
Results may be affected by haemolysis, lipaemia and icterus.
Heparin contamination, including from heparinised lines, may cause spurious results.
This assay is not specific for LMWH, other factor Xa inhibitors will cause interference.
The limit of detection is stated to be 0.04iU/ml.
The linear range is <2iU/ml.
Updated On: 21.05.2025
