Antithrombin (antithrombin III): what it is, what it tells you and when it’s tested

Antithrombin is a protein made by your liver. It acts as a brake on clotting: it blocks thrombin and other factors, so your blood does not clot when it should not. If it is low, the risk of clots in your veins goes up. It is measured in specific cases, usually at the request of a haematologist or a thrombosis clinic.

When it’s tested

Doctors mostly order it after a blood clot at a young age or without a clear cause, when a known antithrombin deficiency runs in the family, or after clots in unusual places.

In hospital it is also measured when heparin does not work as it should: heparin acts through antithrombin, and if antithrombin is low, heparin does less.

It is not a test for people who feel well, not even before starting the pill if nothing runs in the family. That is why it is not in the Rosa panel. Rosa measures albumin in your blood and urine, which says something about your liver and kidneys, which antithrombin depends on too. It also measures platelets.

What it tells you, and what it does not

If it is low, your blood has one less brake against clots. Often the reason is temporary, though: heparin, a clot you have just had, surgery, your liver or your kidneys.

An inherited deficiency is rare: it affects between 1 in 500 and 1 in 5,000 people. It is only confirmed after other causes are ruled out and the test is repeated.

If it is normal, antithrombin deficiency is unlikely. There are other clotting disorders, though, which are looked for with different tests.

That is why it is read together with other tests:

• Albumin: If it is low, your liver or kidneys may be lowering antithrombin too
• Urine albumin (microalbuminuria): If your kidneys leak protein, antithrombin is lost too
• D-dimer: It shows whether there is an active clot, which lowers it
• aPTT: Used to check heparin, which works through antithrombin

Normal antithrombin levels

The lab decides what is normal. Many lab reports give an activity of 80 to 120%. Your own report may print slightly different limits.

It is lower in newborns, and labs use different ranges for the first months of life.

The timing of the test matters: during a clot, on heparin or just after surgery, the value can be low without any deficiency.

What your result means

Find the row for your result, in %. Which values need a doctor? Below 80, or below the limit on your report.

• Below 80: Low. Your blood has less protection against clots. Often there is a passing reason, such as heparin or a recent clot. Rarely it is inherited. Talk to your doctor. They usually have it repeated, away from clots and heparin.
• 80 to 120: Normal. Antithrombin is doing its job as a brake on clotting. Nothing to do.
• Above 120: High. Usually it means nothing. Some anticoagulant tablets can make it look higher. Usually nothing to do. If you take anticoagulants, tell your doctor.

If your antithrombin is high or low

• Low antithrombin: More often caused by heparin, a recent clot, the liver or the kidneys. Rarely inherited. It should be repeated and explained by a doctor.
• High antithrombin: Usually it means nothing.

Why antithrombin can be low

Temporary causes are the most common:

• Heparin: It works through antithrombin and, during treatment, brings it down.
• A recent clot or surgery: Your body uses it up. That is why it is usually not measured during a clot.
• Your liver: It makes antithrombin. If it works less well, it makes less.
• Kidneys that leak protein: With a lot of protein in the urine, some antithrombin is lost too.
• Severe infections and bleeding: In hospital, the body uses it up faster than it makes it.
• An inherited deficiency: It is rare. Usually there have been clots at a young age, in you or your family.

Inherited antithrombin deficiency

It is one of the inherited clotting disorders that raise the risk of clots in the veins the most. It is passed on from one parent, and often several people in the family have had clots.

If you have a known deficiency, the specialists who follow you plan the risky moments with you: surgery, long periods in bed, medicines with oestrogen, pregnancy. At those times, protection with anticoagulants is often needed.

If you are pregnant or trying for a baby and have a known deficiency, talk to your gynaecologist and haematologist early. Do not start or stop anticoagulants on your own.

When to worry

Talk to your doctor if:

• Your antithrombin is below 80%, or below the limit on your report, measured away from clots and heparin.
• You have had a blood clot at a young age or without a clear cause.
• Antithrombin deficiency runs in your family, and you are facing surgery, starting the pill or planning a pregnancy.
• You have a swollen, painful leg, sudden breathlessness or chest pain: call 112 or go to the emergency department.

Frequently asked questions

What are normal antithrombin III levels? Many lab reports give an activity of 80 to 120%. The range on your own report is what counts.

Low antithrombin: what does it mean? That your blood has one less brake against clots. It often comes from heparin, a recent clot, the liver or the kidneys. Your doctor has it repeated to see whether it is a true deficiency.

Should I test antithrombin before taking the pill? Usually not. Guidelines do not recommend these tests for everyone before the pill. They are considered if a known deficiency or blood clots run in your family: talk to your doctor or gynaecologist.

When is the best time to test it? Away from an acute clot and off heparin, if possible. The specialist chooses the right moment.

Does Rosa test antithrombin? No. Rosa measures platelets, and albumin in your blood and urine. If your doctor wants antithrombin, they order it separately, usually with other clotting tests.

Sources

• Van Cott and colleagues, recommendations for clinical laboratory testing for antithrombin deficiency, ISTH. Journal of Thrombosis and Haemostasis, 2020. PMID 31894660
• Patnaik and Moll, inherited antithrombin deficiency: a review. Haemophilia, 2008. PMID 19141163
• Middeldorp and colleagues, American Society of Hematology 2023 guidelines for management of venous thromboembolism: thrombophilia testing. Blood Advances, 2023. PMID 37195076