Test Code Anti-Xa Anti-Xa

Important Note

This is recommended for unfractionated heparin monitoring.

Tube must be properly filled.

Additional Codes

Cerner	NextGen
Anti-Xa	Anti-Xa (Unfractionated Heparin)
UnfractHep Assay(Anti-Xa)

Alternate Name(s)

Unfractionated heparin

Useful For

The Anti-Xa assay is used for the quantitative measurement of plasma levels of unfractionated (UFH) and low molecular weight (LMWH) heparins by measuring their Anti-Xa activity. Heparin therapy is used for the prevention and treatment of thromboembolic disorders. At FMH, only UFH is administered and the measurement of heparin anti-Xa activity is useful for monitoring treatment efficacy.

Methodology

The normal function of a molecule of factor Xa in plasma is to cleave prothrombin to generate thrombin, the enzyme responsible for the formation of the fibrin clot. In the presence of heparin, competition occurs between this mechanism and the inhibitory mechanism exerted by the heparin-antithrombin III complex. This inhibitory mechanism is largely responsible for the anticoagulation action of heparin.

A one-step reaction based on a similar principle is used to measure the Anti-Xa activity.

As soon as factor Xa is added to the plasma-substrate mixture, two opposing reactions take place simultaneously:

Factor Xa hydrolyzes the substrate, and
Factor Xa is inhibited by a heparin-antithrombin III complex, which is made up from heparin and the antithrombin (AT) specific to the patient.

Once the competitive reaction reaches equilibrium, the quantity of paranitroaniline (pNA) that is released is measured and is inversely proportional to the concentration of heparin present in the test medium.

Patient Preparation

None

Collection Instructions

Sample collections must comply with the recommendations for hemostasis tests.
3.2% sodium citrate tubes only.
Tube must be properly filled.
Centrifuge within 1 hour of collection.
Exercise great care in collecting and handling plasma to avoid the release of platelet factor 4 (PF4), which is a potent inhibitor of heparin (UFH).

Specimen Requirements

Preferred Volume	1 mL plasma
Minimum Volume	1 mL plasma
Container	3.2% sodium citrate tube (blue top), BD or Greiner Vacutainer tubes
Stability	Separated Plasma: 2 hours at 15-25⁰ C 2 weeks at -20⁰ C. Separate plasma from cells within 1 hour of collection. Double spin is required: After initial centrifugation and separation into an aliquot tube, centrifuge the aliquot tube and separate plasma from cell button into a new aliquot container.
Rejection Criteria	Clotted Hemolyzed Grossly lipemic (see Limitations section) Expired collection tube Under or Overfilled tube (blood/anticoagulant ratio must be 9:1; tube filled between 90-110 %) Specimens not centrifuged and separated within 1 hour of collection Improperly mixed specimen

Result Reporting and Reference Values

Reference Range	<0.1 IU/mL
Therapeutic Range	0.3 – 0.7 IU/mL
Critical Ranges	>0.7 IU/mL

Reflex Testing