Antithrombin (AT) Activity

TEST: 015040
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CPT: 85300
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Synonyms

  • Antithrombin III Activity
  • Antithrombin III, Functional
  • AT3 Activity
  • Factor Xa Inhibitor
  • Heparin Cofactor Activity
  • Serine Protease Inhibitor

Special Instructions

If the patient's hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted. Refer to Coagulation Collection Procedures for directions.

Expected Turnaround Time

2 - 3 days

Turnaround time is defined as the usual number of days from the date of pickup of a specimen for testing to when the result is released to the ordering provider. In some cases, additional time should be allowed for additional confirmatory or additional reflex tests. Testing schedules may vary.

Related Information

Related Documents

For more information, please view the literature below.

Procedures for Hemostasis and Thrombosis: A Clinical Test Compendium

Specimen Requirements

Specimen

Plasma, frozen

Volume

1 mL

Container

Blue-top (sodium citrate) tube

Collection

Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood-to-anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples unless the sample is collected using a winged (butterfly) collection system. With a winged blood collection set a discard tube should be drawn first to account for the dead space of the tubing and prevent under-filling of the evacuated tube.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternative anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes.

Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.

Storage Instructions

Freeze.

Stability Requirements

Temperature

Period

Frozen

28 days

Freeze/thaw cycles

Stable x3

Patient Preparation

Do not draw from an arm with a heparin lock or heparinized catheter. Patients ideally should not be on unfractionated heparin or direct Xa inhibitor anticoagulant therapy.

Causes for Rejection

Severe hemolysis; improper labeling; clotted specimen; specimen diluted with IV fluids; samples thawed in transit; improper sample type; sample out of stability

Test Details

Use

This test is used for confirmation and characterization of acquired or congenital AT deficiency.

Limitations

AT testing should not be performed on patients on unfractionated heparin therapy because heparin decreases plasma AT levels.6,7 AT results for individuals on warfarin therapy should be interpreted with caution because warfarin can increase levels of AT of patients with heterozygous deficiency into the normal range.7 Thrombin inhibitors, such as dabigatran, hirudin, and argatroban, will not interfere with the assay unless it is based on inhibition of thrombin. Direct Xa inhibitor anticoagulants will interfere with the AT activity assay when it is Xa based, falsely elevating the result, possibly masking a deficiency.6

Methodology

The patient plasma is mixed with heparin and thrombin. The extent of thrombin inhibition is proportional to the AT concentration of the patient sample. Residual thrombin activity is measured as the rate of conversion of a chromogenic substrate and is used to calculate AT concentration.

Additional Information

See Antithrombin (AT) Deficiency Profile [015594] for more clinical information.

Footnotes

1. Adcock DM, Kressin DC, Marlar RA. Effect of 3.2% vs 3.8% sodium citrate concentration on routine coagulation testing. Am J Clin Pathol. 1997; 107(1):105-110. 8980376
2. Reneke J, Etzell J, Leslie S, et al. Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant. Am J Clin Pathol. 1998; 109(6):754-757. 9620035
3. National Committee for Clinical Laboratory Standardization. Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline. 5th ed. Villanova, Pa: NCCLS; 2008. Document H21-A5:28(5).
4. Gottfried EL, Adachi MM. Prothrombin time and activated partial thromboplastin time can be performed on the first tube. Am J Clin Pathol. 1997; 107(6):681-683. 9169665
5. McGlasson DL, More L, Best HA, et al. Drawing specimens for coagulation testing: Is a second tube necessary? Clin Lab Sci. 1999; 12(3):137-139. 10539100
6. Adcock DM, Gosselin R. Direct oral anticoagulants (DOACs) in the laboratory: 2015 review. Thromb Res. 2015 Jul; 136(1):7-12. 25981138
7. Zwicker J, Bauer KA. Thrombophilia. In: Kitchens CS, Alving BM, Kessler CM, eds. Consultative Hemostasis and Thrombosis. Philadelphia, Pa: WB Saunders Co; 2002:181-196.

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
015040 Antithrombin Activity 27811-9 015040 Antithrombin Activity % 27811-9

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