When should antithrombin activity be tested?

The test is indicated for recurrent or unprovoked venous thromboembolism in younger patients, thrombosis in unusual sites, heparin resistance during anticoagulation, family history of clots, and suspected acquired deficiency in critical illness, disseminated intravascular coagulation, or nephrotic syndrome.

Why is timing of the test important?

Antithrombin is consumed during acute thrombosis and cleared more rapidly during heparin therapy, so activity may be spuriously low in these settings. Ideally the test is performed before anticoagulation or repeated weeks to months later to distinguish hereditary from acquired deficiency.

Can heparin affect the result?

Yes. Both unfractionated and low-molecular-weight heparin lower measured antithrombin activity because heparin–antithrombin complexes are cleared more rapidly. The test should be drawn before heparin or at least 48 hours after stopping, though this is not always practical during acute thrombosis.

What does low antithrombin activity indicate?

Activity below 80% suggests deficiency, which may be hereditary (type I quantitative or type II qualitative) or acquired (consumption in thrombosis or DIC, loss in nephrotic syndrome, reduced synthesis in liver disease, drug-induced). Confirmation with repeat testing off anticoagulation is essential.

Is fasting required for an antithrombin test?

No. Fasting is not required, though the test is typically part of a broader thrombophilia work-up that may include other analytes with different preparation instructions.

What other tests are ordered with antithrombin?

Thrombophilia evaluation typically includes protein C and S activity, factor V Leiden and prothrombin G20210A mutation testing, antiphospholipid antibodies, and sometimes factor VIII and homocysteine, depending on the clinical scenario.

How long do antithrombin results take?

Results are typically available within 3–5 business days from specimen receipt; turnaround may be longer if samples are batched or referred to a reference coagulation laboratory.

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Antithrombin (AT) Activity Test

The antithrombin activity test measures functional capacity of the principal endogenous anticoagulant that neutralises thrombin and activated clotting factors. It is ordered to investigate recurrent venous thromboembolism, hereditary thrombophilia, heparin resistance, and acquired deficiency in critical illness or nephrotic syndrome, with results heavily influenced by anticoagulation and acute…

Biomarkers

Blood (plasma)

Sample

3–5 business days

Turnaround

Fasting

What is the Antithrombin Activity test?

The Antithrombin Activity Test measures the functional capacity of antithrombin, the principal endogenous anticoagulant that neutralises thrombin and several activated clotting factors. Practitioners order it to investigate unexplained or recurrent venous thromboembolism, to complete the evaluation of suspected inherited thrombophilia, and to identify acquired deficiency in critical illness, nephrotic syndrome, or disseminated intravascular coagulation. The test is particularly relevant in younger patients with a personal or family history of clots, those who develop thrombosis in unusual sites (cerebral sinus, mesenteric, or portal veins), and patients whose anticoagulation with unfractionated heparin is unexpectedly difficult to achieve. Because both congenital and acquired deficiency affect management — hereditary deficiency may alter duration of anticoagulation and influence family counselling, while acquired deficiency prompts treatment of the underlying disorder — accurate identification of low activity and confirmation with repeat testing off anticoagulation are essential steps in the diagnostic pathway.

What does the Antithrombin Activity test measure?

AT(Antithrombin Activity)

Range: Approximately 80–120% of normal pooled plasma activity; assay-dependent.Optimal: For most clinical purposes, activity between 80 and 120% is considered normal. Values persistently below 80% warrant investigation for hereditary deficiency or an acquired cause, particularly when associated with recurrent thrombosis or a positive family history.% / IU/mL

Antithrombin is a serine protease inhibitor synthesised by the liver that inactivates thrombin and factors IXa, Xa, XIa, and XIIa. Its anticoagulant activity is amplified approximately 1,000-fold by heparin, which binds to a specific pentasaccharide sequence on the molecule and induces a conformational change that accelerates inhibitor-protease complex formation. Antithrombin circulates at relatively high plasma concentrations and represents the principal physiological counterweight to the coagulation cascade, preventing unchecked thrombin generation.

Antithrombin deficiency — whether hereditary or acquired — increases thrombotic risk and may contribute to recurrent venous thromboembolism, arterial events in young patients, or resistance to heparin during therapeutic anticoagulation. The activity assay measures functional capacity rather than immunoreactive protein, capturing both quantitative and qualitative defects, and is therefore the preferred first-line test when antithrombin deficiency is suspected clinically.

Why is the Antithrombin Activity test ordered?

Investigation of recurrent or unprovoked venous thromboembolism, particularly in patients under 50 years with a personal or family history of clots
Work-up of thrombosis in unusual sites (cerebral venous sinus, mesenteric, portal, or hepatic veins) where inherited thrombophilia is more likely
Evaluation of heparin resistance during therapeutic anticoagulation, when standard doses fail to achieve target activated partial thromboplastin time or anti-Xa levels
Screening for hereditary thrombophilia in asymptomatic individuals with a strong family history of thrombosis or known antithrombin deficiency in a first-degree relative
Assessment of suspected acquired antithrombin deficiency in critical illness, disseminated intravascular coagulation, nephrotic syndrome, or severe liver disease
Pre-operative or pre-pregnancy risk stratification in patients with known hereditary deficiency, to guide prophylaxis and antithrombin replacement if needed
Investigation of neonatal purpura fulminans or severe thrombosis in the newborn period, which can reflect homozygous or compound heterozygous deficiency

Sample collection and turnaround

Sample type

Blood (plasma)

Fasting required

Collection method

Venipuncture at patient service centre

Turnaround

3–5 business days

Collection notes

Phlebotomy via LabCorp; sample is collected into sodium citrate (blue-top) tubes. Ideally drawn before starting heparin or at least 48 hours after discontinuation, and several weeks after resolution of acute thrombosis, though clinical circumstances may not permit delay. Plasma should be separated promptly and frozen if not analysed immediately.

Specimen requirements

Sodium citrate plasma (3.2% buffered sodium citrate, 9:1 blood-to-anticoagulant ratio). Minimum volume approximately 1 mL. Separate plasma within two hours of draw and freeze at −20°C or colder if storage exceeds a few hours. Avoid haemolysed, lipaemic, or clotted specimens.

What can affect Antithrombin Activity results?

reducesHeparin therapy (unfractionated or low-molecular-weight)

Heparin binds antithrombin and forms complexes that are cleared more rapidly; plasma activity falls during treatment. Test should ideally be performed before heparin or at least 48 hours after stopping, though this is often not practical in acute thrombosis.

reducesAcute thrombosis and ongoing thrombin generation

Antithrombin is consumed as it neutralises activated clotting factors during active thrombosis or disseminated intravascular coagulation, so activity may be spuriously low during the acute event; confirmation with a repeat sample weeks to months later is required.

reducesOestrogen therapy and pregnancy

Oestrogen-containing oral contraceptives and hormone replacement therapy lower antithrombin modestly; pregnancy also reduces activity, particularly in the third trimester. Interpretation must account for these physiological changes.

reducesSevere liver disease

The liver synthesises antithrombin, so advanced cirrhosis or acute liver failure reduces production. Low activity in this setting is acquired rather than hereditary.

reducesNephrotic syndrome and protein-losing enteropathy

Urinary or enteric loss of plasma proteins, including antithrombin, can cause acquired deficiency; the distinction from hereditary deficiency rests on history and associated findings such as hypoalbuminaemia.

reducesL-asparaginase chemotherapy

Asparaginase inhibits hepatic protein synthesis, causing predictable antithrombin deficiency during treatment; this iatrogenic cause should be recognised when interpreting results in oncology patients.

What do Antithrombin Activity results mean?

Elevated

Mildly elevated antithrombin activity (above 120%) is uncommon and rarely clinically significant; it has been described during acute-phase responses but does not confer a recognised prothrombotic or haemorrhagic risk. Marked elevations are unusual and may reflect assay interference.

Normal

Normal antithrombin activity (80–120%) makes hereditary antithrombin deficiency unlikely but does not exclude other thrombophilias or transient acquired risk factors. Timing of the sample relative to anticoagulation and acute illness is critical — a normal result during heparin or acute thrombosis does not rule out underlying deficiency.

Reduced

Activity below 80% suggests antithrombin deficiency, which may be hereditary (type I quantitative, type II qualitative affecting the reactive or heparin-binding site) or acquired (consumption in acute thrombosis or DIC, loss in nephrotic syndrome, reduced synthesis in liver disease, drug-induced). Hereditary deficiency is associated with a significantly increased lifetime risk of venous thromboembolism; acquired causes require treatment of the underlying condition. Always confirm low results with a repeat sample taken off heparin and remote from acute thrombosis, and interpret alongside functional protein C, protein S, factor V Leiden, and prothrombin G20210A when evaluating thrombophilia.

Note: Antithrombin results are heavily influenced by timing: heparin, acute thrombosis, pregnancy, and critical illness all lower activity, so a single low value during these states cannot distinguish hereditary from acquired deficiency. Repeat testing weeks to months after the acute event and off anticoagulation is essential. Functional activity assays are preferred to immunological antigen assays because they detect both quantitative and qualitative defects. Reference intervals and assay platforms differ between laboratories, and haematology consultation is often warranted to interpret borderline or equivocal results in the broader thrombophilia context.

Frequently asked questions

Antithrombin is a liver-synthesised protein that inactivates thrombin and several clotting factors, serving as the principal endogenous brake on coagulation. Its activity is amplified by heparin, and deficiency — whether inherited or acquired — increases thrombotic risk and may cause heparin resistance.

How to access the Antithrombin Activity test

The Antithrombin Activity reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.

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About this page

Published: October 7, 2026

Last reviewed: October 7, 2026

References:

Bauer KA. Approach to thrombophilia. UpToDate. (2025)
Stevens SM, et al. Antithrombotic therapy for VTE disease: CHEST guideline and expert panel report. Chest. (2021)
Patnaik MM, Moll S. Inherited antithrombin deficiency: a review. Haemophilia. (2008)
British Committee for Standards in Haematology. Investigation and management of heritable thrombophilia. Br J Haematol. (2010)

This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.