Platform
Company
Coagulation
Review status
Currently under review
Pending specialist review and validation.
Antithrombin is a natural protein made by your liver that helps keep blood clotting in balance by limiting enzymes like thrombin and factor Xa. This test measures how well your antithrombin is working in your blood, which is called functional activity. The preliminary version is a rapid screen that can flag a possible problem and may be followed by confirmatory testing if needed.
It is commonly performed as part of a clotting evaluation or when there are concerns about how you are responding to certain blood thinners. The result helps your care team understand whether your body has enough antithrombin function to control unwanted clotting.
Too little antithrombin activity can increase the risk of forming abnormal blood clots. Low activity may be inherited or can occur with conditions such as severe liver disease, major illness, kidney protein loss, widespread clotting, or after surgery. Antithrombin is also important for the effect of heparin, so low activity can make heparin seem less effective.
Your clinician may order this test if you have had a clot at a young age, repeated clots, a strong family history, clots in unusual locations, or an unexpected response to heparin. It can also be checked during evaluations in pregnancy or serious illness when the balance between bleeding and clotting is being assessed.
If your activity is below the laboratory range, your clinician may repeat the test when you are feeling well and not taking certain blood thinners, because illness, surgery, pregnancy, or an active clot can temporarily lower antithrombin. If low levels persist, follow-up testing can look for inherited causes or conditions that reduce production or increase loss, and may include measuring the amount of antithrombin or genetic testing.
A result within the laboratory range generally means your antithrombin function is adequate. Higher-than-expected results are uncommon and usually do not require specific treatment. Regardless of the number, your clinician will interpret the result alongside your history, examination, imaging, and other laboratory tests. If the result could influence anticoagulant choice or dosing, your team will advise on timing for any repeat testing and next steps.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
Active clotting, recent major surgery, sepsis, or widespread clotting can temporarily consume antithrombin and lower the activity result. Retesting after recovery is often recommended.
Heparin requires antithrombin to work and can lower levels during treatment. Warfarin may raise antigen levels, and some direct oral anticoagulants can interfere with certain activity assays. Tell the lab and your clinician about all blood thinners.
Because antithrombin is made in the liver and circulates in the blood, severe liver disease or kidney conditions that cause protein loss can reduce activity results.
Antithrombin activity can be modestly lower during pregnancy and may normalize after delivery. Your clinician may delay definitive testing until the postpartum period if results are borderline.
Newborns naturally have lower antithrombin that increases with age. Some families have inherited antithrombin deficiency, so a persistently low result may prompt testing of relatives.
Blood must be collected into the correct citrate tube and processed promptly. Hemolysis, underfilling, prolonged storage, or incorrect anticoagulant can distort activity results.
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