Skip to content
Blood Test Tracker

Antithrombin Activity

Measures how well antithrombin, a natural brake on blood clotting, is working in your blood.

AI-verified against cited sources. Awaiting expert reviewAlso called AT, AT III, ATIIILOINC 27811-9

Check your result

Sex (optional)

Type your result to see where it falls.

Nothing you type here leaves your browser.

What it is

Antithrombin is a protein made by the liver that helps keep your blood from clotting too easily. It switches off several clotting proteins, mainly thrombin and factor Xa, and the blood thinner heparin makes it work much faster. The test measures its activity as a percentage of normal using a color-change (chromogenic) method. Clinicians use it when someone has had unusual blood clots, when clots run in a family, or when heparin is not working as expected.

Why it matters

Low antithrombin can make blood clots more likely, so it is one of several tests used to look for the cause of unexplained clots. Many low results are temporary, caused by illness, liver or kidney problems, pregnancy or heparin, so a low result is usually repeated before anyone concludes that it is inherited. Inherited antithrombin deficiency is rare.

If it’s low

A lower-than-usual result means your blood's natural clot brake is working less than expected. This is often temporary. Less often it is inherited.

Common causes

  • Late pregnancy and the first weeks after delivery
  • Newborn age, when levels are lower than in adults
  • Heparin treatment
  • A recent or ongoing blood clot
  • Liver disease
  • Kidney disease that leaks protein into the urine (nephrotic syndrome)
  • Severe infection (sepsis) or DIC (disseminated intravascular coagulation)
  • L-asparaginase, a cancer medicine
  • Inherited antithrombin deficiency (rare)

What can help

  • Ask your clinician when the best time to repeat the test is, since heparin, a recent clot and illness can lower the result
  • If it stays low, ask about a follow-up test such as antithrombin antigen, and whether relatives should be tested
  • Before pregnancy, surgery or long travel, mention a confirmed low result to your care team

If it’s high

A result above the usual range is uncommon, and guidelines focus on low antithrombin rather than high. One reason a result can read high is test interference from some blood-thinning pills.

Common causes

  • Direct oral anticoagulant medicines affecting some versions of the test
  • Normal variation between people and between lab methods

What can help

  • Ask your clinician whether a blood thinner you take could affect this test

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 80–130 %. Source: Mayo Clinic Laboratories (Extended Catalog), ARUP Laboratories, British Journal of Haematology, Scientific Reports.

What can affect your result

  • Age: newborns have lower levels, reaching adult values by about 3 months in full-term babies
  • Pregnancy: levels are lower in late pregnancy and the first weeks after delivery
  • Heparin treatment, a recent clot, liver disease, nephrotic syndrome, sepsis and DIC
  • L-asparaginase treatment
  • Direct oral anticoagulants, which can raise some assays
  • Assay type (anti-Xa or anti-IIa based)
  • Heparin treatment can lower antithrombin (Mayo).
  • A recent or ongoing blood clot, liver disease, nephrotic syndrome, severe infection (sepsis), DIC and L-asparaginase treatment can lower the level, so results during acute illness may not reflect your usual level.
  • Direct oral anticoagulants can falsely raise some chromogenic assays: factor Xa inhibitors with anti-Xa-based tests, and thrombin inhibitors with thrombin-based tests.
  • Different chromogenic assays (anti-Xa or anti-IIa based) can give different results for some inherited antithrombin variants.

A snapshot of the level at the time of the draw. Heparin, a recent clot or other illness can lower it temporarily, while an inherited low level stays low.

When to talk to a clinician

Get emergency care right away for signs of a blood clot, whatever your antithrombin result is: a swollen, painful or warm leg or arm, sudden shortness of breath or chest pain, or sudden severe headache, weakness on one side or trouble speaking. Talk with a clinician soon about a result below your lab's range, especially before pregnancy or surgery or if relatives have had blood clots.

Questions you could ask

  • Why was this test ordered for me?
  • Do I need other clotting tests, such as protein C or protein S?
  • Could a blood thinner I take be affecting this result?
  • Could heparin, an illness or pregnancy explain this result?
  • Should we repeat it later, and would an antithrombin antigen test help?
  • Does this result matter for my plans for pregnancy, surgery or travel?

Questions people ask

Does a low antithrombin mean I have an inherited clotting disorder?

Not necessarily. Pregnancy, liver or kidney disease, a recent clot, serious infection and heparin treatment can all lower antithrombin for a while. A low result is usually repeated, and often followed by an antigen test, before anyone decides that the cause is inherited.

Why might my clinician wait before testing after a blood clot?

A fresh clot and the blood thinners used to treat it can change the result. The British Society for Haematology advises testing for natural anticoagulant deficiencies only after 3 months of treatment for an acute clot, because earlier results are hard to trust.

Show the science and sources

Is a change real?

Antithrombin naturally varies about 3.4% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 11% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Systematic review and meta-analysis of within-subject and between-subject biological variation data of coagulation and fibrinolytic measurands (Hollestelle MJ et al.).

For clinicians and researchers

Antithrombin (SERPINC1) is a serpin that inhibits thrombin, factor Xa and other serine proteases, and heparin greatly speeds up that action by binding to a site on antithrombin. The method of choice is a chromogenic assay in the presence of heparin, based on either factor Xa inhibition or thrombin (FIIa) inhibition, and the two designs can differ for variants affecting the heparin-binding site or the reactive site. Mayo reflexes a low activity to an antigen assay, which helps separate type I (quantitative) from type II (qualitative) deficiency. The BSH thrombophilia guideline (2022) lists acquired causes of low activity (liver disease, DIC, nephrotic syndrome, severe sepsis, recent thrombosis, heparin, L-asparaginase, late pregnancy and early postpartum), notes that direct oral anticoagulants can falsely raise some assays, and advises testing only after 3 months of anticoagulation for an acute thrombosis. It reports a roughly 15-fold higher first-VTE risk in heterozygous deficiency, advises against routine testing without a strong family history (two or more first-degree relatives with VTE), and suggests considering testing in pregnancy with a family history or heparin resistance. In one cohort an activity under 70% carried a 3.7-fold higher odds of recurrent VTE.

LOINC 27811-9 (primary), 3174-0. Specimen: plasma.

Sources

  1. Antithrombin Activity, with Reflex to Antithrombin Antigen, Plasma (test AATTF), Mayo Clinic Laboratories (Extended Catalog)
  2. Antithrombin, Enzymatic (Activity) (ARUP test 0030010), ARUP Laboratories
  3. Thrombophilia testing: A British Society for Haematology guideline, British Journal of Haematology (2022)
  4. Comparison of antithrombin activity assays in detection of patients with heparin binding site antithrombin deficiency: systematic review and meta-analysis, Scientific Reports (2023)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

Related markers