Test Catalog

Test ID: HEPTP    
Heparin Anti-Xa, Plasma

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 the 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


This test is not useful for monitoring therapy with the heparinoid "danaparoid."

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); and prolong the activated partial thromboplastin time (APTT). The anti-Xa assay is the preferred method for monitoring low-molecular-weight heparin (LMWH) therapy because of reduced sensitivity of APTT. Heparin is absent in normal plasma. The heparin level obtained has to be analyzed taking into account the treatment given to the patient (type of heparin, dosage, administration mode, time of sampling, etc) and the desired therapeutic effect. It is clinically recommended that platelet counts be monitored frequently in patients receiving unfractionated heparin (UFH) or LMWH in order to detect heparin-induced thrombocytopenia (HIT).

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.

Adult Therapeutic Range

UFH therapeutic range: 0.30-0.70 IU/mL

(6 hours following initiation or dose adjustment)

LMWH therapeutic range: 0.50-1.00 IU/mL for twice daily dosing*

LMWH therapeutic range: 1.00-2.00 IU/mL for once daily dosing*

LMWH prophylactic range: 0.10-0.30 IU/mL

(*sample obtained 4-6 hours following subcutaneous injection)

Interpretation Provides information to assist in interpretation of the test results

Results above the therapeutic range may be supratherapeutic suggesting that the heparin dose may need to be decreased.


Results below the therapeutic range may be subtherapeutic suggesting that the heparin dose may need to be increased.

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

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


In order to reduce the influence from heparin antagonists, such as platelet factor 4 (PF4), dextran sulfate is included in the reaction mixture.


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


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

Clinical Reference Recommendations for 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

7. Hematology: Basic Principles and Practice. Seventh Edition. Edited by R Hoffman, EJ Benz Jr, LE Silberstein, et al. Elsevier, 2018