Platform
Company
Coagulation
Review status
Currently under review
Pending specialist review and validation.
The antithrombin III assay measures how well your antithrombin protein is working to control blood clotting. Antithrombin is a natural anticoagulant made in the liver that helps turn off clotting enzymes like thrombin and factor Xa. This test evaluates the functional activity of antithrombin in your plasma, showing whether it is doing its job effectively.
Doctors use this assay to look for inherited or acquired antithrombin deficiency and to help explain why some people may have an unusual or recurrent blood clot. It can also be used when there is concern that certain medicines that rely on antithrombin are not working as expected.
Antithrombin helps keep clotting in balance. If activity is low, you may be at higher risk of forming clots in veins, such as deep vein clots or clots that travel to the lungs. Finding a deficiency can guide decisions about prevention, treatment length after a clot, and precautions during higher risk times like surgery or periods of limited mobility.
Your clinician may order this test after an unexplained clot, a clot at a younger than typical age, a strong family history of clots, or a clot during pregnancy or after using estrogen-containing medicines. It is also useful when there is suspected resistance to heparin, or in conditions that can reduce antithrombin levels, such as liver disease, kidney disorders that cause protein loss, or widespread activation of clotting.
Results are reported as antithrombin activity compared with the expected range for healthy individuals. Lower than expected activity can suggest a deficiency, which may be inherited or acquired due to illness, liver dysfunction, kidney protein loss, or active clotting. Normal activity suggests the protein is functioning adequately. Higher than expected activity is uncommon and is usually not clinically significant on its own.
Because illness, surgery, pregnancy, and certain anticoagulant medicines can temporarily change antithrombin activity or interfere with the assay, your clinician may recommend repeat testing when you are well and off specific medications. If activity is low, follow-up may include an antithrombin antigen level to help distinguish type of deficiency, review of other clotting tests, and possibly genetic testing based on your history. Even with a confirmed deficiency, many people do well; your care plan will be tailored to your personal and family history of clots and your risk during specific situations.
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.
Unfractionated heparin and low molecular weight heparin can lower measured antithrombin activity through consumption and can interfere with some assays. Testing is often timed after these drugs are cleared.
Some chromogenic antithrombin activity methods can be affected by direct factor Xa or direct thrombin inhibitors, which may falsely alter results. Tell the lab and your clinician about all anticoagulants.
During an active clot, major surgery, sepsis, or disseminated intravascular coagulation, antithrombin can be consumed and appear low. Repeating the test after recovery can clarify whether a true deficiency exists.
Antithrombin is made in the liver and can be reduced in liver dysfunction. Kidney disorders with heavy protein loss can also lower levels by urinary loss of antithrombin.
Antithrombin activity can be modestly lower during pregnancy, and estrogen-containing therapies can influence clot risk. Timing of testing and interpretation should account for these factors.
Improper tube filling, difficult blood draws, or delays in processing can affect clotting assays. Using the correct citrate tube and prompt processing helps ensure reliable results.
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