Antithrombin III test
Quick answer: This test measures antithrombin, a natural protein that stops blood from clotting too much, as its activity or its amount. It is ordered after unexplained blood clots, with a strong family history of clots, or when heparin does not work as expected.
- SampleBlood
- TakesAbout 5 minutes
- Before the testNo special preparation is usually needed; tell staff about any blood thinners you take, as they can affect the result.
- In fertility careInherited antithrombin deficiency raises the risk of clots in pregnancy, so it may be checked in women with a past clot or strong family history.
How the test is done
- 1Blood is drawn into a light-blue citrate tube
- 2The tube is mixed gently and sent to the lab
- 3The lab measures how well your antithrombin blocks clotting enzymes
- 4Results are given as a percentage of normal activity
What your results can mean
Reading your report
Recent clots, liver disease and some medicines can lower levels, so a low result is often repeated; discuss it with your doctor.
If the result is outside the usual range
A low level can point to inherited antithrombin deficiency, which raises clot risk, or can be seen with liver disease, nephrotic syndrome, heparin treatment or a recent clot. High levels are not usually a concern.
Who usually has this test
People with unexplained clots, a family history of clots, or a poor response to heparin.
Risks and aftercare
Bruising is possible; keep taking any blood thinner as prescribed.
Names and options
Also called: antithrombin III, AT III, antithrombin activity
Options
- Activity
- Antigen (amount)
Listed by the hospital as
- ANTI THROMBIN ACTIVITY, PLASMA III
- ANTI THROMBIN ANTIGEN, PLASMA III
Questions about this test
When should antithrombin be tested?
It is usually tested away from a recent clot and not during heparin treatment, as these can lower the level. Your doctor will plan the timing.
Can antithrombin deficiency affect pregnancy?
Yes, it raises the risk of clots in pregnancy. Women with it are usually cared for by a specialist team.
Is antithrombin deficiency inherited?
It can be inherited from one parent. Family members may be offered testing.
Related tests
Tamara Hospital & IVF Centre, Rajajinagar
34/3, 10th Cross Road, opp. Vardhaka Sardar Patel School, 1st N Block, 2nd Stage, Rajajinagar, Bengaluru 560010
The hospital is open 24 hours. Call before you come to check the timing for your scan or sample collection.
Get directions ↗IVF Pulse fertility care at Tamara Hospital →
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.