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UW Health SMPH
American Family Children's Hospital

UW Hospital and Clinics Lab Test Directory

Test Name: Heparin Level by anti-Xa

Test Code(s): HEPRN / XLMWH

CPT Code(s): 85520

Methodology: Chromogenic Anti-Xa Assay

Clinical Significance: Note:  This is NOT a Factor X assay.
The plasma level of unfractionated heparin (UFH) or low molecular weight heparin (LMWH) is determined by the measurement of anti-Xa activity.

Days Performed: Daily, 24 hours.

Turnaround Time: Stat: 2 hours.
Routine: 1 working day.

Specimen Requirements

Specimen: Blood

Collection Instructions: Do NOT draw through a line containing heparin (this assay measures any type of heparin present in the sample, including line contamination). The venipuncture MUST be atraumatic to avoid hemolysis and contamination with tissue factors and platelets. Tube must be allowed to fill completely.  

Please indicate patient history and type of heparin the patient is receiving. For Low Molecular Weight Heparin (LMWH), collect specimen as close as possible to 4 hours post-dosing.

If unable to collect 1.8 ml tube (pediatrics only) please call Core Coagulation Lab at 263-9617 or Special Coagulation Lab at 263-5005 for alternate tube and instructions.

Collection Container: Preferred: Light blue top (3.2% NaCitrate)

Collection Volume: Preferred: Full 3.5 mL tube
Pediatrics: Pediatric tube filled to fill mark

Sample Analyzed: Plasma

Volume Required: Preferred: 1 mL
Pediatrics: 0.3 mL

Specimen Processing: Centrifuge specimen to yield platelet poor plasma (platelet count of plasma should be less than 10 K/L). Separate plasma and transfer to plastic tube or vial. Freeze plasma at -20C or below.

Specimen Transport: Transport whole blood specimen at room temperature to UWHC Core Laboratory (B4/220) within 1 hour of collection. See Specimen Processing for instructions if transport criteria are not met. Transport frozen plasma aliquot on dry ice to UWHC Core Laboratory (B4/220).

Unacceptable Criteria: Hemolyzed specimens, clotted specimens, partially filled tubes and whole blood specimens greater than 1 hour old when received by the laboratory are not acceptable.

Stability: Ambient: 1 hour off cells
Refrigerated: Not acceptable
Frozen: 2 weeks at -20C; 6 months at -70C


Expected Results:

If sample was collected through a line, this method is unable to distinguish heparin level in the sample from possible line contamination.


Recommended therapeutic ranges:

Standard (unfractionated) heparin: 0.3 - 0.7 anti-Xa IU/mL


Low molecular weight heparin (LMWH): varies with the type and manufacturer, but is typically between 0.5-1.1 anti X-a IU/mL. For LMWH, collect specimen as close to FOUR hours post-dosing as possible. Recommended therapeutic range is valid for twice- daily dosing. Levels will be higher for once daily dosing.


Reference: Supplement to Sept 2004, Chest, vol 126, number 3.


Contact UWHC Special Coagulation at (608) 263-5005 for more information.

Additional Information:

NOTE: This is NOT a Factor X assay. The test is carried out with an excess of purified antithrombin to ensure compensation for any existing deficiency of this protein. Level determined using same type of heparin as patient is receiving.

Heparin level determination  anti xa

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