Heparin assay (Anti Xa assay)
The anti FXa assay is the method of choice when monitoring low molecular weight (LMW) heparin therapy, and can be a replacement for the APTT test when monitoring therapy with unfractionated heparin. Anti-Xa may be used to monitor some patients who have ‘heparin resistance’ (ie. do not respond as expected to UFH) or who have an underlying condition or interfering factor(s) that alter the APTTR test result.
Low molecular weight heparin (LMWH) therapy is usually not monitored, but it may be necessary to monitor anti-Xa in some cases, such as in patients who are pregnant, obese, very young, elderly and those who have kidney disease or dysfunction (given that LMWH is primarily cleared from the body by the kidneys). Any condition that decreases kidney function can potentially decrease LMWH clearance, increasing its concentration in the blood and increasing the potential for excessive bleeding.
Monitoring heparin (especially LMWH) therapy in patients who are pregnant, obese, very young or elderly and those with renal impairment.
Special handling: avoid prolonged stasis during venepuncture.
Sample must be received by lab within 4 hours of collection.
Please note: samples will be rejected if underfilled or overfilled.
Record last updated
August 20, 2025
Sample Container
2.7 ml Sodium citrate Vacutainers (Light blue top)
Sample Container
1.8ml Sodium Citrate Paediatric Tube (Light blue top)
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Test Results
Yes, we have a Pathology User Guide
Yes. Please email your request to Imperial.nwlpcustomerservice@nhs.net
Results are communicated to clinical staff and are normally not communicated direct to patients either on telephone requests or in written report. If you need to contact us please click here to view the contact details page
Please contact the Customer Service Team Monday to Friday from 8:30am – 6:00pm on 0203 313 5353. Alternatively, see the Pathology User Guide for how to contact the relevant department. If advice is not urgent you can also email your query to Imperial.nwlpcustomerservice@nhs.net
Additional tests may be added by discussion with the relevant laboratory. The time limit for adding additional tests to a sample already received in the laboratory will depend on the type of sample and the department it was sent to.
Information on the repertoire of tests performed by pathology, including reference ranges, turnaround times and specimen requirements, can be found in the test directory.
Phlebotomy services are not managed by NWLP. For further information please see:
Imperial College Healthcare Trust Phlebotomy services
Chelsea and Westminster Hospital NHS Foundation Trust Phlebotomy services
NWLP operates UKAS accredited medical laboratories as per the references below. The current scope of accreditation for all NWLP’s laboratories can be found on the UKAS website.
See the following UKAS ref numbers:
- Clinical Biochemistry Ref: 8673
- Haematology and Blood Transfusion Ref: 8674
- Infection & Immunity Sciences (Including Immunology, Virology, Microbiology and Histocompatibility and Immunogenetics ) Ref: 8659
- Cellular Pathology (Including Histopathology, Cytology and Molecular Pathology) Ref: 9615
If you require copies of our certificate of accreditation and associated documentation please contact Imperial.nwlpcustomerservice@nhs.net.
UKAS requires the laboratories to be accredited for a particular repertoire/scope (ISO15189), any changes or additions to repertoire require assessment by UKAS (extension to scope).
The BD Hospital Tube Guide, BD GP Tube Guide and BD Paediatric Tube Guide provide visual references to the correct tube types and collection instructions.
Home testing
INSTRUCTION SHEET FOR BLOOD SAMPLE COLLECTION
We also have two instruction videos available (one with subtitles and one without) – please click on the links to view the videos on Google.
Our accreditations
Upholding excellence in diagnostic standards
Our laboratories are accredited by the UK Accreditation Service (UKAS) against the international standard ISO15189:2022. UKAS is the accreditation body for the UK that assesses medical laboratories.
Find out more about our accreditations on the UKAS website.
UKAS website