Sign in →

Test Code UFHXA Unfractionated Heparin Anti-Xa Assay, Plasma

Important Note

The Unfractionated Heparin Anti-Xa assay should only be ordered when monitoring levels of unfractionated heparin.

If monitoring levels of Low Molecular Weight Heparin, please order the Low Molecular Weight Heparin Anti-Xa assay.

Additional Codes

Epic Order Code: LAB3473
Sunquest Order Code: UFHXA

Specimen Required

Container Type:

1 solid blue top tube or 1 clear blue top tube filled to line:

Optimal Collection Volume: 

Tube must be filled to line.  Solid blue top tube requires 2.7 mL whole blood, clear blue top tube requires 1.8 mL whole blood.

Collection Instructions:

1. Do not draw from an arm with a heparin lock.

2. If collected through a blood collection set (butterfly),
a discard tube must first be drawn to eliminate dead space from

3. Tube must be filled to proper level as indicated by opaque
line etched on each blue-top tube. Volume of blood is 2.7
mL in full draw (solid, light-blue top) or 1.8 mL for partial draw
(clear and blue top). 

4. Gently invert 5 to 10 times to mix blood.

Processing Instructions: 

1. Sample must be centrifuged within one hour of collection.

2. If testing will be performed >4 hours from collection time, sample should be processed to obtain platelet poor plasma, aliquoted and frozen.

Useful For

Measuring heparin concentration:

  • In the presence of prolonged baseline activated partial thromboplastin time (APTT) (eg, lupus anticoagulant, "contact factor" deficiency, etc)
  • When unfractionated heparin dose needed to achieve desired APTT prolongation is unexpectedly higher (>50%) than expected

Not useful for monitoring therapy with the heparinoid "danaparoid"

Profile Information

Test ID   Reporting Name Available Separately Always Performed
UFHXA Unfractionated Heparin Anti-Xa assay Y Y

Method Name

The Liquid Anti-Xa kit (ACL TOP) is a 1-stage chromogenic assay based on a synthetic chromogenic substrate and on factor Xa inactivation.(Package insert HemosIL Liquid Anti-Xa)

Stability Information

Specimen Type Temperature Time
Whole blood Room temp 1 hour
Platelet Poor Plasma Ambient  4 hours
Platelet Poor Plasma Frozen  4 weeks

Rejection Due To

Hemolysis Mild OK; Moderate Reject; Gross Reject
Lipemia Mild OK; Moderate OK; Gross Reject
Icterus Mild OK; Moderate OK; Gross Reject
Other Sample refrigerated
  Sample clotted
  Sample not filled to line on tube


Specimen Minimum Volume

Collecting minimum volumes can result in a need for sample recollection, and/or a delay in results. Minimum volumes are subjective and cannot account for all aspects of specimen and testing needs. Refer to the Specimen Required section for optimal volumes for laboratory specimens. Contact the Bronson Laboratory if complex collection exceptions occur that require more information.


Minimum Volume: 1.8mL whole blood in clear blue top tube

Performing Laboratory

Bronson Laboratory, Coagulation- Kalamazoo, Battle Creek

Day(s) and Time(s) Performed

Monday through Sunday 24/7

Reference Values

UFH Therapeutic Range:

0.30-0.70 IU/ML for treatment of DVT/PE

0.30-0.50 IU/ML for cardiac/neuro protocols

CPT Code Information


Sample Retention Time

1 day