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Test Code HEPA Unfractionated Heparin Assay by Anti - Xa

Specimen Type



Light blue top tube, 3.2% sodium citrate

Preferred Volume

2.7 mL

Offsite Collection Instructions

  • Samples must be centrifuged at >1500 x g for 10 minutes at room temperature.
  • Carefully remove the plasma from the cells using a plastic transfer pipette avoiding the buffy coat.
  • Dispense the plasma into a plastic tube (glass is not acceptable) and centrifuge the plastic tube for a second centrifugation at >1500 x g for 10 minutes at room temperature.
  • Remove the top portion of the plasma being careful to leave a small amount of plasma at the bottom of the tube undisturbed (about 250mL). Then, aliquot the removed plasma into 2 plastic tubes containing minimum of 500 uL of specimen each.
  • The plasma samples must then be frozen at -20ºC.
  • Once solidly frozen they must be shipped on dry ice. Samples that have thawed in transit will be rejected.


Note: For clients who do not have access to dry ice do not freeze the specimen. Follow the centrifugation instructions or send whole blood sample. It now becomes vital that the centrifuged specimen arrives in the lab within four hours of blood collection. Whole blood must arrive within one hour of collection.

UCMC Collection Instructions

Samples can be collected at any time. Test is performed daily from 8:00 am to 10:30 pm. Samples that arrive after 10:30 pm will be processed, frozen and tested the next morning.

Please note that testing for unfractionated heparin level may be added on to an existing order only up to one hour following the time of blood draw due to technical considerations of this assay.


Specimens may be collected and sent to the laboratory at any time. Testing is performed 7 days a week from 8:00 am to 10:30 pm. Samples received after 10:30 pm will be run the following day.

Turnaround Time

4 hours

STAT Availability

Not Available

Test Methodology


Additional Information

All 3.2% citrate tubes require a ratio of 9 parts blood to 1 part anticoagulant (Example: a 3.0 mL tube requires 2.7 mL blood added to 0.3 mL of anticoagulant).

Unfractionated heparin level is monitored by functional anti-Xa assay.
For monitoring of heparin therapy the frequently employed target range is 0.3-0.7 anti-Xa Units/mL (e.g., for treatment of venous thrombosis). For the lower intensity anticoagulation range of aPTT 50-70 seconds (e.g., as in guidelines from American College of Chest Physicians), the     corresponding range is currently 0.13-0.41 anti-Xa Units/mL.

CPT Code


Minimum Volume

2.7 mL

Fee Code




Unfractionated heparin by anti-Xa

Heparin level

Heparin level by anti-Xa