PTD - 9

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Test Directory -  Anti Xa assay


Anti Xa, Low Molecular Weight Heparin, LMW, LMH

Clinical Indications

.Patients on Low Molecular Weight Heparin (LMWH), Danaparoid (Oxaron), Fondaparinux (Arixtra) therapy. Anti Xa usually not monitored,  may be requested in some cases. These include patients who are pregnant, obese, very young , elderly and those who have kidney disease or dysfunction. Normally requested as a peak test 3-4 hours after administration of anti Xa agent.

Test Includes

Anti Xa activity

Request Form 

Combined Pathology Blood form (Yellow/Black or Blue for GP's)


On request. When requesting please state on form if patient is on Danaparoid or Fondaparinux 

Specific Criteria

  After discussion and agreement with Consultant Haematologist.

Turnaround Time

Same day


 Citrated whole blood




 2 x Blue capped vacutainer tubes


  Samples must be received by lab within 2 hours of collection.

Lab. Handling


Causes for Rejection

Inadequately labelled, unlabelled, haemolysed, clotted or lipaemic samples. Sample received more than 2 hours after collection.


  Sample haemolysed, lipaemic, under filled or more than 2 hours old

Reference Range

  A therapeutic anti-Xa level for LMWH is 0.5-1.0 IU/mL 4 hours after a dose of LMWH.
A prophylactic anti-Xa level for LMWH is 0.1-0.3 IU/mL 4 hours after a dose of LMWH. Danaparoid: 0.5-0.8 U/mL


Reference ranges for anti-Xa levels depend on heparin type, dose, schedule and indication. When a person is not taking heparin, anti-Xa concentrations should be zero or undetectable.

When it is used as to monitor LMWHs, anti-Xa levels are usually ordered as a 'peak' test. It is collected 3-4 hours after a LMWH dose is given, when the concentration of LMWH in the blood is expected to be at its highest level. Random and “trough” anti-Xa tests may also be ordered when there are concerns that a LMWH may be accumulating e.g. in renal failure.

Version 1.0 / January 2013