Also found in: Acronyms.


A gene on chromosome 1q23-q25.1 that encodes antithrombin III, a member of the serine protease inhibitor (serpin) family that rapidly inhibits thrombin, as well as other activated serine proteases of the coagulation system, and regulates the coagulation cascade.

Molecular pathology
SERPINC1 mutations cause antithrombin-III deficiency.

Antithrombin III

Synonym/acronym: Heparin cofactor assay, ATIII.

Common use

To assist in diagnosing heparin resistance or disorders resulting from a hypercoagulable state such as thrombus.


Plasma (1 mL) collected in a completely filled blue-top (3.2% sodium citrate) tube. If the patient’s hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted.

Normal findings

(Method: Chromogenic Immunoturbidimetric)
AgeConventional Units (% of Normal)
1–4 days39–87%
5–29 days41–93%
1–3 mo48–108%
3–6 mo73–121%
6–12 mo84–124%
1–5 yr82–139%
6–17 yr90–131%
18 yr-older adult80–120%


Antithrombin III (AT-III) can inhibit thrombin (factor IIa) and factors IX, X, XI, and XII. It is a heparin cofactor, produced by the liver, interacting with heparin and thrombin. AT-III acts to increase the rate at which thrombin is neutralized or inhibited, and it decreases the total quantity of thrombin inhibited. Patients with low levels of AT-III show some level of resistance to heparin therapy and are at risk for venous thrombosis. AT III deficiency can be acquired (most common) or congenital.

This procedure is contraindicated for



  • Investigate tendency for thrombosis

Potential diagnosis

Increased in

  • Acute hepatitis
  • Renal transplantation (Some studies have demonstrated high levels of AT III in proximal tubule epithelial cells at the time of renal transplant. The exact relationship between the kidneys and AT III levels is unknown. It is believed the kidneys may play a role in maintaining plasma levels of AT III as evidenced by the correlation between renal disease and low AT III levels.)
  • Vitamin K deficiency (decreased consumption related to impaired coagulation factor function)

Decreased in

    Carcinoma (related to decreased synthesis) Chronic liver failure (related to decreased synthesis) Cirrhosis (related to decreased synthesis) Congenital deficiency Disseminated intravascular coagulation (related to increased consumption) Liver transplantation or partial hepatectomy (related to decreased synthesis) Nephrotic syndrome (related to increased protein loss) Pulmonary embolism (related to increased consumption) Septic shock (related to increased consumption and decreased synthesis due to hepatic impairment) Venous thrombosis (related to increased consumption)

Critical findings


Interfering factors

  • Drugs that may increase AT-III levels include anabolic steroids, gemfibrozil, and warfarin (Coumadin).
  • Drugs that may decrease AT-III levels include asparaginase, estrogens, heparin, and oral contraceptives.
  • Hematocrit greater than 55% may cause falsely prolonged results because of anticoagulant excess relative to plasma volume.
  • Incompletely filled collection tubes, specimens contaminated with heparin, clotted specimens, or unprocessed specimens not delivered to the laboratory within 1 to 2 hr of collection should be rejected.
  • Placement of the tourniquet for longer than 1 min can result in venous stasis and changes in the concentration of the plasma proteins to be measured. Platelet activation may also occur under these conditions, resulting in erroneous measurements.

Nursing Implications and Procedure


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching:  Inform the patient this test can assist in diagnosing clotting disorders.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s hematopoietic system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues,  as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications: N/A
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture. Fill tube completely. Important note: When multiple specimens are drawn, the blue-top tube should be collected after sterile (i.e., blood culture) tubes. Otherwise, when using a standard vacutainer system, the blue top is the first tube collected. When a butterfly is used, due to the added tubing, an extra red-top tube should be collected before the blue-top tube to ensure complete filling of the blue-top tube.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis.


  • Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include antibodies cardiolipin, echocardiography, lung perfusion scan, aPTT, procalcitonin, protein C, protein S, US venous Doppler extremity studies, venography lower extremities, and vitamin K.
  • See the Hematopoietic System table at the end of the book for related tests by body system.


antithrombin III, a natural α-globulin coagulation inhibitor.
References in periodicals archive ?
This press release contains forward-looking statements as defined in the Private Securities Litigation Reform Act of 1995, including without limitation statements about the prospects for developing ATIII and the opportunities inherent in Genzyme Transgenics Corporation's other development programs.
Tests include PT, aPTT, TT, FIB, heparin, LMWH, intrinsic and extrinsic pathway factors, ATIII, protein C and S, vWF, and D-dimer, as well as calibration and quality control.
A standard 405 nm light filter permits various coagulation and fibrinolysis tests such as PT, aPIT, TT, FIB, heparm, protein C and S, ATIII, venom time, APCR, and factor assays.
The binding of heparin to ATIII results in an increase in thrombin inhibitory activity.
Genzyme Transgenics and Genzyme General have established a joint venture, ATIII LLC, for the development, marketing, and distribution of rhATIII in the United States and Europe.
In the treatment group, ATIII blood levels increased to 103 percent of normal 30 minutes after the start of surgery, and were at 104 percent of normal at the end of CPB.
ATIII deficiency is a key factor in heparin resistance, since heparin requires ATIII for effective anticoagulation.
Actual results may differ materially depending on many factors, including without limitation the likelihood of regulatory and other approvals of the Cell Genesys acquisition, Genzyme Molecular Oncology's ability to complete preclinical development of its products and manufacture sufficient quantities of aaATIII to conduct clinical trials, the content and timing of decisions made by the FDA with respect to its products, the enrollment rate of clinical trials, the actual timing and results of clinical trials, Genzyme Molecular Oncology's future spending and actual cash needs, and the ability of Genzyme Molecular Oncology and the ATIII LLC to negotiate and execute a definitive collaboration agreement.
Mice with these tumors were treated with daily injections of the different conformations of ATIII.
Genzyme Molecular Oncology and the ATIII LLC have agreed to equally share in the development costs of an aaATIII cancer therapy and equally share in any profits from a successful oncology product created through the collaboration.
In the phase II trial, Genzyme Transgenics administered ATIII to patients undergoing coronary artery bypass grafting (CABG), a type of open heart surgery.