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Anti-factor Xa activity

Coagulation · also called Anti-Xa, Heparin level, LMWH level

What it measures

Measures how strongly plasma inhibits added factor Xa, reflecting the level of heparin or other Xa inhibitors. Used to monitor low-molecular-weight heparin, unfractionated heparin and sometimes rivaroxaban or apixaban.

Specimen and preparation

Specimen: Citrated plasma (3.2% sodium citrate)
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Preparation: For LMWH, draw a peak sample 3–4 h after a subcutaneous dose. Centrifuge within 1 h for heparin.

Reference range

Group	Range
No anticoagulant	<0.1 IU/mL
UFH therapeutic	0.3–0.7 IU/mL
LMWH twice daily, peak	0.5–1.0 IU/mL
LMWH prophylaxis, peak	0.2–0.5 IU/mL

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

Heparin or LMWH overdose
Renal impairment with LMWH accumulation
Direct Xa inhibitor present

Causes of low values

Underdosing
Antithrombin deficiency (heparin resistance)
Sample drawn at wrong time

Method

Chromogenic assay calibrated for the specific drug

Good to know

Helpful in pregnancy, obesity, renal failure and children. DOAC levels need drug-specific calibrators and are reported in ng/mL.

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