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.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.

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.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners
References in periodicals archive ?
As we know, preoperative ATIII activity plays a crucial role in the development of PVST and was found to be an independent predictor of PVST after splenectomy [27].
LMWH can suppress factor Xa by combining with ATIII to depress the activation of thrombin and formation of thrombosis so that it can be used for regular anticoagulation [50].
After a network meta-analysis of different anticoagulation strategies for splenectomy, we found that application of ATIII was the best and most suitable for cirrhotic patients.
Ademas, la union de la trombina a la TM hace que esta proteasa sea mas sensible a la inhibicion por la ATIII, en comparacion con la trombina libre.
En contraste, la deficiencia congenita de la mayoria de las proteinas anticoagulantes como ATIII, PC y PS esta asociada a la aparicion de un estado de hipercoagulabilidad, protrombotico o trombofilico, sin embargo, en estos casos no se observan alteraciones en las pruebas clasicas, por lo que estas no se emplean en el estudio de pacientes con trombofilias primarias (51-54).
En el caso de la ATIII, esta prueba se basa en la funcion antitrombinica de este inhibidor en presencia de heparina (70,71).
New chromogenic ATIII activity kit which is insensitive to heparin cofactor II and designed for use on automated instruments [abstract].
Some of Affinity Biologicals Inc.'s polyclonal antibodies include Plasminogen, Prothrombin, ATIII, PAI-1, Kininogen, Protein C, Protein S, Vimentin and Alpha 2- Macroglobulin.
Using photo-optical detection with a 405 nm halogen light source, the compact, benchtop analyzer performs many tests, including: PT, aPTT, TT, FIB (fibrinogen), factor assays, INR, ATIII, plasminogen, [alpha]-2 antiplasmin, heparin anti-Xa, protein C, APCR (activated protein C resistance), lupus screen and confirmatory, PNP, P+P, and mixing studies, as well as user-programmable tests such as clottable protein C and S.
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.