Test Overview

Test Methodology


Test Usage

AT3 levels are used as a possible indicator of hypercoagulability, and to test for the hereditary deficiency of antithrombin. AT3 levels might also be of use in cases of suspected heparin failure especially prior to heparinization.

Reference Range*

75 - 125%

* Reference ranges may change over time. Please refer to the original patient report when evaluating results.

Test Limitations

If a patient is on heparin therapy or has received heparin within the last 10 days, the results of this test may be falsely decreased. The AT3 level may be diminished for several days after surgery or a thrombotic event. Warfarin therapy has been reported to raise AT3 levels.

Test Details

Days Test Performed


Analytic Time

1 hour

Soft Order Code


MiChart Code

Antithrombin III



Antithrombin III Assay, Functional
Serine Protease Inhibitor
Factor 10a Inhibition

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Specimen Requirements

Offsite Collection Instructions

Collect specimen in a blue top (citrate 3.2%) tube. Mix by inversion. Specimen should arrive at lab within 3 hours of collection; transport at room temperature. Alternatively, centrifuge, aliquot plasma into a plastic vial, and freeze the specimen within 4 hours of collection. Transport frozen specimen on dry ice. Collection of the blood through lines that have been previously flushed with heparin should be avoided. If the blood must be drawn through a VAD (vascular access device), the line should be flushed with 5 mL of saline and the first 5 mL of blood or six dead space volumes of the VAD discarded.

Rejection Criteria

Specimens will be rejected if not properly filled, clotted, grossly hemolyzed, or contaminated with heparin


Test is not indicated for patients on heparin, low-molecular-weight heparin, or direct thrombin inhibitor therapy.


Blue Top Tube

Normal Volume

Full 2.7mL tube or 1 mL plasma aliquot

Minimum Volume

Full 1.8mL tube or 1 mL plasma aliquot

Additional Information

Individuals with hereditary AT3 deficiency will show resistance to anticoagulation with heparin but can be anticoagulated with warfarin. Decreased AT3 levels may be found in women taking oral contraceptives, in conditions associated with \"consumption\" of coagulation proteins (pulmonary embolism, thrombophlebitis, acute myocardial infarction, and DIC), and in liver disease.

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