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ANTI Xa (089-0539)
Test Mnemonic:


  STAT: 1½  hour; ROUTINE: 4 hours

Specimen Requirements:

Blue-topped (3.2% sodium-citrate) tube. Routine venipuncture; discard 1st mL of blood by collecting a discard tube prior to collecting the blue-topped (3.2% sodium-citrate) tube.  For collections with butterfly blood collection sets, a discard tube should also be collected prior to collection of the blue top to ensure sufficient sample volume. Drawing a discard tube will displace the air from the blood collection set tubing to ensure proper blood draw volume.


 Blue-topped (3.2% sodium-citrate) tube


If delivery time is to be greater than 1 hour from time of draw, centrifuge the specimen for 15 minutes at 2000-2500g, and separate the plasma from the cells.

·         Transfer plasma to 12x75 plastic tube

·         Cap tube and re-spin  for 15 minutes at 2000-2500g

·         CAREFULLY remove from centrifuge without disturbing any platelets and/or cell pellets that might be on the bottom or sides of the tube.

·         Carefully transfer plasma into freezer tubes (plastic screw-top cryo-tubes) for testing or freezing. (Note:  Testing for Unfractionated Heparin cannot be done on frozen plasma)

Specimen Preparation:

Samples should be received in laboratory within ½ hour of draw for best results.    Must clearly mark requisition with type of Heparin (e.g. Unfractionated, Low Molecular Weight (Lovenox)). Notify Lab (409-772-2293) 1 hour  prior to specimen collection

This test is not currently available in-house for patients receiving a direct Xa inhibitor (e.g. rivaroxaban, apixaban, or edoxaban).  It is available as a send-out test.


Plasma is stable for 2 hours at 20+ 50C. Plasma for testing Low Molecular Weight Heparin (Lovenox) is stable frozen for 1 week at -20C.

Causes for Rejection:

QNS, clotted, severely hemolyzed specimen, plasma specimen greater than 2 hours old, wrong tube type, high hematocrit (> 55%), sample identification error, sample processed/transported, and/or stored improperly.

Reference Range:

Unfractionated Heparin (UFH)  therapeutic range: 0.3 - 0.7 IU/mL)

Low Molecular Weight Heparin (LMWH) therapeutic range: 0.6 - 1.0 IU/mL (for twice daily administration) 

Turnaround Time:

Chromogenic and measured photometrically at 405nm.


Samples are accepted 24 hours per day at Sample Management, 7.412 CSW Bldg.

Clinical Indication:

Quantitatively determines the plasma level of Unfractionated Heparin (UFH), or Low Molecular Weight Heparin (LMWH).

CPT 4 Code:



An antithrombin deficiency will lead to an underestimation of the heparin level.

When ordering tests for which Medicare or Medicaid reimbursement will be sought, physicians should only order tests that are medically necessary for the diagnosis or treatment of the patient. Components of the organ or disease panels may be ordered individually. The diagnostic information must substantiate all tests ordered and must be in the form of an ICD-10 code or its verbal equivalent.
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