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Test Code LAB4139 ANTI-Xa ASSAY, LMWH

Specimen Type

Blue top tube (buffered 3.2% liquid sodium citrate) filled to the indicator with whole blood or Sodium Citrate Plasma removed from cells.

Specimen Volume

3mL Blue top (3.2% Sodium Citrate)
5mL Blue top (3.2% Sodium Citrate)
1mL Blue top (3.2% Sodium Citrate)
500uL (minimum) Sodium Citrate Plasma spun and removed from cells.Blue top tube (buffered 3.2% liquid sodium citrate) filled to the indicator with whole blood or Sodium Citrate Plasma removed from cells.

Turnaround Time

STAT: 1 hour
Routine: 2 hours

Test Schedule

Daily

Sample Stability

Room temp: 1 hour unspun, 2 hours spun. Platelet poor plasma is required.
Frozen samples are stable for 2 weeks at -20C or 12 months at -70C.

Method

AntiXa Chromogenic

Reference Ranges

LMWH therapeutic ranges:

       0.50-1.00 IU/mL for twice daily dosing*

       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

LMWH Critical range:

       Greater than 2.00

 

Synonyms

Anti-Xa; LMWH Assay;

Anti-Factor XA Assay;

Anti Xa; Lovenox;

Enoxaparin; Fragmin;

Low Molecular Weight Heparin

CPT Codes

85520

Specimen Processing

Samples should be centrifuged and run within one hour. If time interval between drawing and testing exceeds 1 hour, centrifuge specimen, separate plasma, centrifuge again, separate into plastic tube and freeze at -20C or less.

Collection Instructions

Fill a Blue Top Container to the Indicator mark on the tube. Overfilled and underfilled tubes are not acceptable.

Rejection Criteria

Severely hemolyzed specimens, clotted samples

Clinical Information

The anti-Xa test is used to quantitatively determine the plasma levels of unfractionated heparin (UFH) and low molecular weight heparin (LMWH) by measuring the anti-Xa activity on antithrombin in a competitive system using a synthetic chromogenic substrate. UFH and LMWH are used for prevention and treatment of thromboembolic diseases. The quantitative determination of these levels in plasma are useful for monitoring treatment efficacy.

 

The normal function of a molecule of factor Xa, as soon as it appears in plasma, is to cleave its natural substrate, prothrombin, to generate thrombin, the enzyme responsible for the fibrin clot formation. In the presence of heparin, competition occurs between this mechanism and the inhibitory mechanism exerted by the heparin-antithrombin III complex, made up of heparin and antithrombin (AT) from the patient. This inhibition is largely responsible for the action of heparin.

 

As soon as factor Xa is added to the plasma-substrate mixture, two reactions occur simultaneously: hydrolysis of the substrate by factor Xa and inhibition of factor Xa by the heparin-antithrombin III complex. After the necessary period for the competitive reaction to reach equilibrium, the quantity of para-nitroaniline (pNA) released is inversely proportional to the concentration of heparin present in the plasma.