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Anti Xa*






Heparin Anti-Xa; Antifactor Xa; Heparin Assay 

Specimen Type



Blue-top sodium citrate tube.

Collection Instructions

Blood is to be collected in a blue-top  tube containing 3.2 % buffered sodium citrate. Evacuated collection tubes must be filled to completion to ensure a proper blood to anticoagulant ratio. Sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of specimen and anticoagulant. Place specimen on ice to deliver to laboratory.


2ml. plasma (not blood volume)
Minimum volume: 1ml. plasma
Minimum volumes are the same regardless of age.

Additional Information

Determine the plasma level of unfractionated heparin (UFH) or low molecular weight heparin (LMWH). Historically, many laboratories have monitored heparin levels using the PTT. A large number of conditions can complicate the use of the PTT in monitoring therapy. In these cases, the anti-Xa assay may be more appropriate for monitoring heparin therapy.

Patient Preparation

Heparin anti-Xa levels should be drawn 6 hours after initiation of unfractionated heparin therapy, or change in dose, whereas with low molecular weight heparin, levels should be drawn 6 hours after administration if once daily and 3-4 hours if twice daily.

Specimen Processing Instructions

Once delivered to the laboratory on ice, the specimen is centrifuged ASAP and the plasma carefully removed and frozen.

Transport Requirements


Specimen Stability


Rejection Criteria

Specimens not received in lab on ice; quantity not sufficient; blue top tube not filled properly;gross hemolysis.

Reference Range

Reference Range accompanies report

Critical Value


Test Includes

Plasma level of heparin. 


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.Standard curves are created using multiple concentrations of UFH and LMWH and are used to calculate concentration in the patient plasma.

Testing Frequency




Reviewed By

Lisa McBride 

Reviewed Date



* Denotes reference test*

Reviewed by Lisa McBride on February 05, 2018

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