Anti-XA Assay
Haematology Overview / Directory
Purpose of the test
A chromogenic assay used for the quantitative determination of low molecular weight heparin (LMW heparin). Unfractionated heparin can also be measured by this assay but is not usually required.
The majority of patients on LMW heparin will not require monitoring, but where calculation of the dose by body weight is unreliable regular anti-Xa assays may be required e.g. pregnancy, obesity, infants.
The biological activity of heparin lies in its ability to accelerate the inhibitory effect of antithrombin (AT) on the coagulation proteases. The amount of heparin is determined from the anti-Xa activity expressed by the [AT - Heparin] complex formed in plasma
Reference Range
Heparin results are reported as activity (IU/ml).
Therapeutic ranges:
Prophylaxis 0.20 – 0.40 IU/ml
Treatment of thrombotic event 0.50 – 1.00 IU/ml
NB. These are guidelines only ; each patient must be individually assessed according to their clinical condition
Sample Requirements
Coagulation citrated sample with minimal arm stasis, adequately filled with no haemolysis or clots. It is important to ascertain which preparation of LMW heparin the patient is on. Samples should be taken 3-4 hours after the injection is given and the sampling time noted on the request form.
Storage and Transport
Frozen aliquots of plasma may be sent on ice together with verification of time of collection and time of separating.
Turnaround Time
The assay is run every in batches every week
Results may be available same day by prior arrangement.
Time Limit for Extra Tests
1 hour
Factors affecting results or interpretation
Clots of any size , haemolysis, underfilling or overfilling will affect result.
Sample to be taken 4-6 hours post subcutaneous injection of LMWH.
Contacts
Haemostasis Laboratory on 020 3299 9000 ext 2434
e-mail: liz.ford@nhs.net
