Heparin Anti-Xa

CPT: 85520
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Test Details


  • Heparin Assay


Determine the plasma level of unfractionated heparin (UFH) or low molecular weight heparin (LMWH)


The heparin anti-Xa assay is based on the ability of heparin to inhibit the activity of activated factor X (Xa) in the reagent. The reagent includes excess antithrombin, making the heparin in the sample the rate-limiting reagent for Xa inhibition. Heparin in the patient sample inhibits the enzymatic conversion of a Xa-specific chromogenic substrate to colored product by factor Xa. Standard curves are created using multiple concentrations of UFH and LMWH and are used to calculate concentration in the patient plasma.

Reference Interval

Reference intervals indicate therapeutic levels.10

• Unfractionated heparin (UFH): 0.3−0.7 IU/mL

• Low molecular weight heparin (LMWH): 0.4−1.1 IU/mL bd

• Low molecular weight heparin (LMWH): 1.0−2.0 IU/mL peak

− Twice daily dosing: 0.6−1.1 IU/mL

− Once daily dosing: 1.0−2.0 IU/mL

• Fondaparinux: 0.39−0.50 mg/0.50 mg/mL

Additional Information

UFH and LMWH are anticoagulants commonly used to decrease the risk of venous or arterial thrombosis.6-9 Overdosing with these medications can increase the risk of hemorrhage and inadequate dosing decreases the efficacy of anticoagulation. These drugs work as anticoagulants by enhancing the ability of plasma antithrombin to bind and inactivate the serine proteases XIIa, XIa, IXa, Xa, and thrombin. Therapeutic monitoring is commonly performed because of the wide interindividual variation in response to this therapy.

Historically, many laboratories have monitored heparin levels using the activated partial thromboplastin time (aPTT) test. A large number of conditions can complicate the use of the aPTT in monitoring UFH therapy.6 The College of American Pathologists (CAP) divided these factors into three groups.6 The first group of complicating factors encompasses those that affect the bioavailability of heparin. Aging, obesity, changes in heparin binding proteins, hepatic disease, renal disease, and heparin resistance fall under this heading. The second group of complicating factors includes those that alter the aPTT dose response to heparin. Increased factor VIII or fibrinogen levels, decreased antithrombin levels, or a mild reduction in multiple factors (as might be seen in the early stages of a consumptive coagulopathy or in oral anticoagulant therapy) are examples of this type of complicating factor. Lastly, those factors that tend to cause a prolonged aPTT in the absence of heparin therapy should be considered. Lupus anticoagulants or deficiency in contact factors can produce an extended baseline aPTT. In all of these cases, the heparin anti-Xa assay may be more appropriate for monitoring heparin therapy.

The aPTT should not be used for therapeutic monitoring of low molecular weight heparin because this drug typically does not affect the aPTT significantly.6,7

Specimen Requirements


Plasma, frozen


2 mL

Minimum Volume

1 mL


Blue-top (sodium citrate) tube


Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood to anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples, except when using a winged blood collection device (ie, "butterfly"), in which case a discard tube should be used.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternate anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes. Centrifuge and carefully remove the plasma using a plastic transfer pipette, being careful not to disturb the cells. Transfer the plasma into a LabCorp PP transpak frozen purple tube with screw cap (LabCorp N° 49482). Freeze immediately and maintain frozen until tested. To avoid delays in turnaround time when requesting multiple tests on frozen samples, please submit separate frozen specimens for each test requested.

Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.

Storage Instructions


Causes for Rejection

Gross hemolysis; clotted specimen; frozen specimen thawed in transit; improper labeling

Clinical Information

Special Instructions

If the patient's hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted. Refer to Coagulation Collection Procedures for directions. Heparin anti-Xa levels should be drawn six hours after initiation of unfractionated heparin therapy or change in dose, whereas, with low molecular weight heparin, levels should be drawn six hours after administration when given once daily and three to four hours when administered twice daily. Fondaparinux levels should be measured three hours postadministration.


1. Adcock DM, Kressin DC, Marlar RA. Effect of 3.2% vs 3.8% sodium citrate concentration on routine coagulation testing. Am J Clin Pathol. 1997 Jan; 107(1):105-110.8980376
2. Reneke J, Etzell J, Leslie S, Ng VL, Gottfried EL. Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant. Am J Clin Pathol. 1998 Jun; 109(6):754-757.9620035
3. National Committee for Clinical Laboratory Standardization. Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline. 5th ed. Villanova, Pa: NCCLS; 2008. Document H21-A5:28(5).
4. Gottfried EL, Adachi MM. Prothrombin time and activated partial thromboplastin time can be performed on the first tube. Am J Clin Pathol. 1997 Jun; 107(6):681-683.9169665
5. McGlasson DL, More L, Best HA, Norris WL, Doe RH, Ray H. Drawing specimens for coagulation testing: Is a second tube necessary? Clin Lab Sci. 1999 May-Jun; 12(3):137-139.10539100
6. Olson JD, Arkin CF, Brandt JT, et al. College of American Pathologists Conference XXXI on laboratory monitoring of anticoagulant therapy: Laboratory monitoring of unfractionated heparin therapy. Arch Pathol Lab Med. 1998 Sep; 122(9):782-798.9740136
7. Hirsh J, Anand SS, Halperin JL, Fuster V, American Heart Association. Guide to anticoagulant therapy: Heparin: A statement for healthcare professionals from the American Heart Association. Circulation. 2001 Jun 19; 103(24):2994-3018.11413093
8. Hirsh J, Warkentin TE, Shaughnessy SG, et al. Heparin and low-molecular-weight heparin: Mechanisms of action, pharmacokinetics, dosing, monitoring, efficacy, and safety. Chest. 2001 Jan; 119(1 Suppl):64S-94S.11157643
9. Francis CW, Berkowitz SD. Antithrombotic and thrombolytic agents. In: Kitchens CS, Alving BM, Kessler CM, eds. Consultative Hemostasis and Thrombosis. Philadelphia, Pa: WB Saunders Co; 2002:375-391.
10. http://www.practical‐haemostasis.com/Miscellaneous/Miscellaneous%20Tests/anti_xa_assays.html. Accessed on 05/08/2017


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
117101 Heparin Anti-Xa 3274-8 117102 Heparin Anti-Xa IU/mL 3274-8

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