Pathology Handbook

Anti-Xa assay


Xa assay, Low Molecular weight heparin assay

Clinical Indications

Monitoring of prophylactic and therapeutic doses of LMWH is not required routinely. However, anti-Xa assay may be of use in individuals for whom the standard weight adjusted dose may be unreliable. These include patients with renal failure, the obese, the pregnant, neonates and infants, those with unexpected bleeding on LMWH.

Samples for peak levels of anti-Xa activity should be taken 4-6 hours after subcutaneous administration. For trough levels the sample taken 24 hours after administration may be informative. 

Note: this is NOT the same as a Factor X assay

Request Form 

Request via ICE


Monday-Friday 9am-5pm (Tests recieved outside this time will be frozen for processing on the next routine day)

Specific Criteria

Patient should be bled with minimal stasis

Patient Preparation

Patient should ideally be at rest

Turnaround Time

Same day.


Fresh citrated Blood (sample MUST reach the laboratory within 1 hour of collection)


4.5 ml


Blue capped vacutainer tube

Vacutainer blue top


Clean venepuncture; sample transported to laboratory immediately after collection

Causes for Rejection

Underfilled, clotted or haemolysed samples

Reference Range

Therapeutic level: 0.35 - 0.7 anti-Xa units/mL