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Test ID HEPTP Heparin Anti-Xa, Plasma

Useful For

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."

Method Name

Chromogenic Method

Reporting Name

Heparin Anti-Xa, P

Specimen Type

Plasma Na Cit

Ordering Guidance

To test for the presence of inhibitors directed against factor X (FX), order 10INE / Factor X Inhibitor Evaluation, Plasma.

Specimen Required

Specimen Type: Platelet-poor plasma

Collection Container/Tube: Light-blue top (3.2% sodium citrate)

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL

Collection Instructions:

1. For complete instructions see Coagulation Guidelines for Specimen Handling and Processing.

2. Centrifuge, aliquot plasma, and centrifuge plasma again.

4. Aliquot plasma into a plastic vial leaving 0.25 mL in the bottom of centrifuged vial.

Additional Information: Double-centrifuged specimen is critical for accurate results as platelet contamination may cause spurious results.

Specimen Minimum Volume

0.5 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Plasma Na Cit Frozen (preferred) 14 days
  Ambient  2 hours

Reject Due To

Gross hemolysis Reject
Gross lipemia OK
Gross icterus OK

Clinical Information

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; 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 must be analyzed in the context of 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 or LMWH in order to detect heparin-induced thrombocytopenia.

Reference Values

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 to 6 hours following subcutaneous injection


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.


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, 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

1. Marci CD, Prager D. A review of the clinical indications for the plasma heparin assay. Am J Clin Pathol. 1993;99(5):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(8 Pt 1):1223-1230

3. Jeske W, Messmore HL Jr, Fareed J. Pharmacology of heparin and oral anticoagulants. In: Loscalzo J, Schafer AI. Thrombosis and Hemorrhage. 2nd ed. Williams and Wilkins; 1998:1193-1204

4. Monagle P, Michelson AD, Bovill E, Andrew M. Antithrombotic therapy in children. Chest. 2001;119(1 Suppl):344S-370S

5. Fraser G, McKenna J. Monitoring low molecular weight heparins with antiXa activity: house of cards or firm foundation? Hospital Pharmacy. 2003;38(3):202-211. doi:10.1177/001857870303800302

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(6):1064-1083

7. Hoffman R, Benz Jr EJ, Silberstein LE, et al. Hematology: Basic Principles and Practice. 7th ed. Elsevier; 2018

Method Description

The Liquid Anti-Xa kit (ACL TOP) is a 1-stage chromogenic assay based on a synthetic chromogenic substrate and on factor Xa inactivation.(Package insert: HemosIL Liquid Anti-Xa. Instrument Laboratory; R5, 12/2020)

Day(s) Performed

Monday through Sunday

Report Available

Same day/1 day

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
HEPTP Heparin Anti-Xa, P 3274-8


Result ID Test Result Name Result LOINC Value
HEPTP Heparin Anti-Xa, P 3274-8