Heparin assay (Anti-Xa)


Alternative names, keywords

Low molecular weight heparin (LMWH), LMW heparin, Factor Xa assay, unfractionated heparin (UFH).

Samples required

Trisodium citrate blood (blue cap, 3.5 mL tube) taken with minimal stasis, filled to the line. Smaller tubes are available for paediatric samples.

Samples must be analysed or processed in the laboratory within 4 hours of venepuncture.

Blood should be taken 4-6 hours after administration of heparin. This test can only be added to a request within 2 hours of sample collection.

Test indications

For monitoring LMW heparin or unfractionated heparin (UFH). It is essential to state the type of heparin.

Reference range

No clinically defined reference ranges exist in the literature. The stated ranges are consensus opinion based upon discussion with other exemplar sites and after discussion with the Lead Haemostasis Consultant. Please contact a Consultant Haematologist should further advice on dosing be required.


Target anti-Xa range

Enoxaparin/UFH prophylaxis

0.2 - 0.4 units/mL

Enoxaparin treatment - once daily dosing

0.5 - 1 units/mL

Enoxaparin treatment - twice daily dosing

0.5 - 1 units/mL

Intravenous unfractionated heparin infusion  

0.3 - 0.7 units/mL

Note change to reference ranges from April 2020.

Turnaround time

Typically within 2 hrs, although non-urgent samples may be frozen for subsequent testing.


Haematology (Coagulation, ext. 64306 (RSCH) or ext. 68201 (PRH))