Antithrombin activity is a measure of how effectively a protein in your blood called antithrombin works to prevent unwanted clot formation. Normal antithrombin activity falls between 80% and 120%, and levels below that range can increase your risk of dangerous blood clots. This protein is one of the body’s most important natural anticoagulants, and testing its activity helps identify people who may need closer monitoring or treatment to prevent clotting events.
How Antithrombin Works in the Body
Antithrombin is produced in the liver and acts as a natural brake on your clotting system. When blood vessels are injured, your body activates a chain of clotting proteins to seal the wound. Antithrombin keeps that process in check by binding to and neutralizing several of those clotting proteins, most importantly thrombin (the enzyme that converts liquid blood into a solid clot) and factor Xa (a key step earlier in the clotting chain). It also has some effect on other clotting factors further upstream.
Without enough functioning antithrombin, the clotting system can become overactive, forming clots inside blood vessels where they aren’t needed. These clots most commonly appear as deep vein thrombosis in the legs or as pulmonary embolism when a clot travels to the lungs.
Antithrombin also has a direct relationship with heparin, one of the most widely used blood-thinning medications. Heparin doesn’t thin the blood on its own. Instead, it works by dramatically amplifying antithrombin’s natural ability to neutralize clotting factors. This means that if your antithrombin activity is too low, heparin may not work properly, a situation known as heparin resistance. This is particularly relevant during cardiac surgery and other procedures where heparin is critical.
Activity Test vs. Antigen Test
When doctors order antithrombin testing, they may order two different types. The antithrombin activity test (also called a functional assay) measures how well the protein does its job of inhibiting thrombin. The antithrombin antigen test measures how much antithrombin protein your body has produced, regardless of whether it’s working correctly. The activity test is considered the primary diagnostic tool because it captures both quantity and function in a single number.
These two tests together help distinguish between different types of deficiency. If both results are low, the body isn’t making enough antithrombin. If the antigen level is normal but activity is reduced, the body is producing a normal amount of protein, but the protein itself is structurally flawed and doesn’t function properly.
What Causes Low Antithrombin Activity
Low antithrombin activity has two broad categories of causes: inherited and acquired.
Inherited Deficiency
Hereditary antithrombin deficiency is rare, affecting roughly 1 in 3,000 people. It comes in two forms. In type I deficiency, the body simply doesn’t produce enough antithrombin, so both the antigen level and activity are reduced equally. In type II deficiency, the body produces a normal quantity of antithrombin, but the protein is dysfunctional, so the antigen test looks normal while the activity test comes back low. People with congenital deficiency typically have activity levels in the 40% to 60% range.
An activity level below 70% in the context of a family history of blood clots suggests congenital antithrombin deficiency. Because the condition is inherited, family members of an affected person may also benefit from testing.
Acquired Deficiency
Several medical conditions can lower antithrombin activity temporarily. Liver disease is a common cause, since the liver is where antithrombin is manufactured. Kidney disease, particularly nephrotic syndrome (a condition that causes the kidneys to leak large amounts of protein into the urine), can deplete antithrombin because the protein literally spills out of the body. Research in patients with proteinuria found antithrombin in the urine of the majority of patients tested, with a strong correlation between the amount of protein lost in urine and how low antithrombin levels dropped. Severe infections, particularly sepsis, can also consume antithrombin rapidly as the clotting system becomes widely activated.
Why It Matters During Pregnancy
Pregnancy already increases clotting risk due to hormonal and circulatory changes, and antithrombin deficiency compounds that risk significantly. In a study of women with inherited type I antithrombin deficiency, 11.6% of pregnancies in women who did not receive preventive blood-thinning treatment resulted in a venous clotting event. Women with a positive family history of clots faced an even higher rate of 11.8%, while those without a family history had a lower but still meaningful risk of 5.4%.
Women who received preventive treatment with a low-molecular-weight blood thinner during pregnancy had a clotting rate of 7.0%, suggesting some protective benefit, though the numbers in the study were small. For women who know they have antithrombin deficiency, pregnancy planning with a hematologist can help determine the best approach to reducing clot risk.
When Testing Is Recommended
Antithrombin activity testing is not a routine screening test. The American Society of Hematology recommends thrombophilia testing for patients with venous blood clots, particularly those who are young, have had recurrent clotting episodes, have clots in unusual locations (such as the abdomen or brain), or have a family history of clotting disorders.
Timing matters for accurate results. Testing during an active clot, while on heparin, during pregnancy, or while taking certain hormonal medications can temporarily lower antithrombin levels and produce misleading results. Your doctor will typically wait until you’re in a stable state before ordering the test.
How Low Antithrombin Activity Is Managed
Having low antithrombin activity doesn’t automatically mean you need treatment. If your levels are above 70% and you’ve never had a clot or other risk factors, replacement therapy isn’t indicated. Management depends on how low your levels are, whether you’ve had clotting events, and whether you’re facing a high-risk situation.
For people with congenital deficiency, antithrombin concentrate (a purified form of the protein given intravenously) is reserved for specific high-risk moments: major surgery, delivery, trauma, or during treatment of an active clot until oral blood thinners reach effective levels. It’s used temporarily alongside heparin rather than as a long-term therapy. The dosing is individualized, with each unit per kilogram of body weight expected to raise plasma antithrombin activity by about 1.5%, and levels are monitored every 12 to 48 hours to guide ongoing doses.
For acquired deficiency, the focus is on treating the underlying condition. If liver disease is driving the deficiency, improving liver function will typically restore antithrombin levels. In severe sepsis with widespread clotting activation, antithrombin concentrates may improve outcomes, though the evidence supporting this use remains limited. In these cases, the concentrates are given at high doses and without heparin, which differs from the approach used in congenital deficiency.