Test Catalog

Interpretive Handbook

Test 80609 :
Heparin Anti-Xa Assay, Plasma

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Heparins are sulphated glycosaminoglycans that inactivate thrombin, factor Xa, and several other coagulation factors; act by enhancing activity of the plasma coagulation inhibitor, antithrombin III (AT III); prolong the activated partial thromboplastin time (APTT); of low molecular weight heparin have minimal effect on the APTT, and require an alternative method, such as the anti-Xa assay, to monitor therapy.

Useful For Suggests clinical disorders or settings where the test may be helpful

Measuring heparin concentration:

-In patients treated with low molecular weight heparin preparations

-In presence of prolonged baseline activated partial thromboplastin time (APTT) (eg, lupus anticoagulant, "contact factor" deficiency, etc.)

-When unfractionated heparin dose needed to achieve desired APTT prolongation is unexpectedly higher (>50%) than expected

Interpretation Provides information to assist in interpretation of the test results

Therapeutic ranges:

Adults > or =18 years

-Standard heparin (UFH): 0.30 to 0.70 IU/mL

-Low molecular weight heparin (LMWH): 0.50 to 1.00 IU/mL for twice daily dosing (specimen drawn 4-6 hours following subcutaneous injection) or 1.00 to 2.00 IU/mL for once daily dosing (specimen drawn 4-6 hours following subcutaneous injection)

Children <8 weeks

-Standard heparin (UFH): 0.30 to 0.70 IU/mL

-LMWH: 0.50 to 1.00 IU/mL (specimen drawn 4-6 hours following subcutaneous injection)

-LMWH (prophylactic): 0.10 to 0.30 IU/mL

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

This test should not be used for routine monitoring of unfractionated heparin (UFH) therapy except as indicated above. The activated partial thromboplastin time (APTT) test is routinely used to monitor UFH therapy.


This is not a test for the presence of inhibitors directed against factor X (FX).


Plasma specimen must be depleted of platelets by repeat centrifugation before freezing.


During freezing and thawing, residual platelets in plasma release platelet factor 4, possibly causing falsely-low heparin levels.


Not useful for monitoring therapy with the heparinoid "danaparoid."


Very low endogenous antithrombin III (AT III) levels might result in spuriously-low results.  


Antibodies to bovine FX or AT III can interfere with assay (very rare).


Lipemia, icterus, and hemolysis may interfere with color detection and give inaccurate results.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

> or =18 years

UFH therapeutic range: 0.30-0.70 IU/mL

LMWH therapeutic range:

0.50-1.00 IU/mL for twice daily dosing

1.00-2.00 IU/mL for once daily dosing

(sample obtained 4-6 hours following subcutaneous injection)


> or =8 weeks-17 years

UFH therapeutic range: 0.30-0.70 IU/mL

LMWH therapeutic range: 0.50-1.00 IU/mL (sample obtained 4-6 hours following subcutaneous injection)

LMWH prophylactic range: 0.10-0.30 IU/mL


<8 weeks

Reference values have not been established for patients who are less than 8 weeks of age.

Clinical References Provides recommendations for further in-depth reading of a clinical nature

1. Marci CD, Prager D: A review of the clinical indications for the plasma heparin assay. Am J Clin Pathol 1993;99:546-550

2. Houbouyan L, Boutiere B, Contant G, et al: Validation of analytical hemostasis systems: measurement of anti-Xa activity of low-molecular-weight-heparins. Clin Chem 1996;42:1223-1230

3. Jeske W, Messmore HL Jr, Fareed J: Pharmacology of heparin and oral anticoagulants. In Thrombosis and Hemorrhage. Second edition. Edited by J Loscalzo, AI Schafer. Baltimore, MA, Williams and Wilkins, 1998, pp 1193-1204

4.  Monagle P, Michelson AD, Bovill E, Andrew M: Antithrombotic therapy in children. CHEST 2001;119:344-370

5.  Fraser G, McKenna J: Monitoring low molecular weight heparins with antiXa activity: house of cards or firm foundation? Hospital Pharmacy 2003;38:202-211

6.  Nutescu EA, Spinler SA, Wittkowsky A, Dager WE: Low-molecular-weight heparins in renal impairment and obesity: available evidence and clinical practice recommendations across medical and surgical settings. Ann Pharmacother 2009;43:1064:1083