Coagulation Panel Updated Apr 17, 2026

Antithrombin III (AT III)

Antithrombin III (AT III) is a blood protein that helps regulate clot formation by slowing specific clotting factors. On an Antithrombin III blood test, the result reflects how much AT III is present or how well it is working in the sample. AT III on a lab report is usually reviewed with other coagulation markers to understand the overall clotting picture.

What Is Antithrombin III (AT III)?

Antithrombin III (AT III) is a protein in blood that helps control clotting by inhibiting several clotting factors. It is measured in a coagulation panel, so AT III on a lab report reflects the amount or activity of this clot-regulating protein in the sample. AT III is often reported as a percentage or as an activity value, depending on the AT III test method used.

Why Is Antithrombin III (AT III) Tested?

AT III is tested in a coagulation panel and in the AT III test when a lab wants a closer look at clotting balance. It helps describe how the blood’s clotting system is behaving alongside other coagulation markers. AT III on a blood test is often used as part of a broader review of clotting-related proteins, especially when a more complete picture is needed.

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Antithrombin III (AT III) Normal Range

Group Range Unit
Adult Male 80–120 %
Adult Female 80–120 %

Reference ranges may vary by laboratory and individual factors.

What Does High AT III Mean?

A high AT III result means there is more antithrombin activity or concentration than expected in the sample. In many labs, a result above about 120% may be considered high AT III, although the Antithrombin III normal range depends on the assay. When AT III on a lab report is high, the result may reflect concentration changes in the blood sample or differences in how the test was run.

Associated factors

Dehydration — reduced plasma volume can make AT III look higher by concentrating the sample.
Recent strenuous exercise — temporary shifts in blood volume can affect the measured AT III.
Altitude exposure — lower plasma volume at higher altitude can increase measured concentration.
Smoking — can change blood chemistry and may influence AT III results.
Hormone therapy — some medications can alter clotting protein levels and AT III activity.
Pregnancy — normal shifts in blood proteins can change AT III values.
Age-related variation — AT III reference values can shift slightly across adult groups.
Test method differences — activity assays and antigen assays may report different AT III values.

What Does Low AT III Mean?

A low AT III result means there is less antithrombin activity or concentration than expected in the sample. In many labs, a result below about 80% may be considered low AT III, though the normal Antithrombin III range depends on the laboratory method. On an Antithrombin III blood test, low values suggest reduced clot-regulating protein in the measured sample.

Associated factors

Low protein intake — reduced overall protein availability can contribute to lower AT III.
Recent blood loss — loss of blood proteins can reduce the measured AT III level.
Fluid overload — extra plasma volume can dilute AT III in the sample.
Liver-related protein changes — the liver makes AT III, so reduced production can lower results.
Certain medications — some drugs can affect AT III activity or measured concentration.
Major inflammation — protein patterns in blood can shift and lower AT III values.
Ongoing clotting activity — AT III can be used up faster when clotting is active.
Lab handling issues — sample timing or processing can affect the AT III test result.

How Antithrombin III (AT III) Relates to Other Values

AT III is read with other coagulation markers to show the broader clotting pattern. Common related values include prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrinogen, and D-dimer. When AT III is reviewed with PT and aPTT, the pattern helps describe how fast clotting pathways are moving. AT III on a lab report can also be compared with platelet count (PLT) to show whether the blood has enough clot-related components overall.

What Factors Affect Antithrombin III (AT III) Levels?

AT III can vary with age, sex, hydration, and recent physical activity. AT III on a lab report may also differ by time of day, since blood volume and protein levels can shift slightly over the day. Pregnancy and hormone use can change AT III levels, and lab methods can also affect the reported value. Ethnicity and altitude may influence reference patterns in some populations. The same sample can read differently across laboratories if the AT III test uses a different assay or reporting unit.

How It Is Tested

AT III is measured from a blood draw, usually from a venous sample. The lab may measure AT III activity or AT III concentration, and results are commonly reported as a percentage or as units per volume depending on the assay. The AT III test may use clot-based or antigen-based methods.

How to Prepare

No fasting is usually required for an Antithrombin III test. AT III on a blood test may still vary with recent activity, hydration, or medication timing.

Frequently Asked Questions

What is the normal range for Antithrombin III?
The Antithrombin III normal range is commonly about 80% to 120%, but the exact range depends on the lab method. On an AT III test, results are often reported as a percentage of activity. The same AT III value can be interpreted differently if the assay uses a different reference interval.
What does AT III stand for?
AT III stands for Antithrombin III. It is a blood protein measured on a coagulation panel. On a lab report, AT III is usually shown as a percentage or activity value.
What does a high Antithrombin III mean on a lab report?
A high AT III result means the measured antithrombin level or activity is above the lab’s reference interval. In many settings, high AT III is reported when the value is above roughly 120%. The result can reflect blood concentration changes or assay differences rather than a single fixed pattern.
What does a low Antithrombin III mean on a lab report?
A low AT III result means the measured antithrombin level or activity is below the expected range. In many labs, low AT III is below about 80%. On an Antithrombin III blood test, this can reflect dilution, protein loss, or reduced production of the protein.
Can hydration affect Antithrombin III?
Yes, hydration can affect AT III. Dehydration can concentrate the sample and make high AT III more likely on paper, while extra fluid can dilute the value and make low AT III more likely. Small changes in hydration can matter when the result is near the reference range.
What is the difference between Antithrombin III and fibrinogen?
Antithrombin III helps slow clotting, while fibrinogen is a clot-building protein. On a blood test, AT III shows clot regulation, and fibrinogen shows how much clotting material is available. The two values are often read together in a coagulation panel.
What unit is Antithrombin III measured in?
AT III is commonly measured in percent (%) when the lab reports activity. Some AT III tests may also use units related to concentration, depending on the method. The unit should be read together with the reference range on the lab report.
How much can Antithrombin III change between tests?
AT III can change a little between tests because of hydration, recent activity, and lab method differences. Small shifts of a few percentage points are possible even when the overall pattern is stable. Bigger changes usually reflect a real change in the sample or a different assay.
Is Antithrombin III different for men and women?
AT III reference values are often similar for men and women in adults, but some laboratories may list slightly different ranges. The Antithrombin III normal range should always be matched to the lab that ran the AT III test. Pregnancy can also shift the value compared with a nonpregnant adult baseline.
Why is Antithrombin III tested in a coagulation panel?
AT III is included in a coagulation panel to show part of the blood’s clot-regulating system. It helps explain the overall clotting balance when reviewed with PT, aPTT, fibrinogen, and related markers. On an AT III on a blood test result, the value adds context to the rest of the panel.

Disclaimer

This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.

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