Antithrombin III: normal range, high and low

In shortAntithrombin III is a protein that limits clotting by inactivating thrombin and factor Xa. A typical reference is 80–120% of normal activity. Low values increase the risk of venous thrombosis; high values have little clinical meaning. The result is read together with the rest of the coagulation panel.

Reference ranges

Group	Reference range
Functional activity — typical range (international reference, not a national standard)	80–120%

Reference from the Labcorp test directory (international reference, not a national standard). Each laboratory sets its own interval — the range printed on the report applies.

Check your value

Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.

A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.

What this marker is

Antithrombin is made in the liver and circulates in the blood, where it neutralizes the enzymes that drive clot formation. Heparin works by dramatically speeding up this natural brake — which is why antithrombin levels matter for heparin therapy.

Testing is ordered after unexplained or recurrent blood clots, clots at a young age, a family history of thrombosis, or when heparin seems less effective than expected.

When it is high

No clinical concernHigh antithrombin activity is not associated with disease and needs no follow-up by itself.

When it is low

Inherited antithrombin deficiencyA genetic cause of lifelong increased clotting risk; family members may be offered testing.

Acquired deficiencyLiver disease, nephrotic syndrome, acute thrombosis, and heparin therapy itself can lower the level temporarily.

What can skew the result

These factors can shift the value without any disease — worth considering before a retest:

Heparin therapyAcute thrombosisLiver diseaseNephrotic syndromePregnancy

How to prepare

Blood from a vein. When possible, testing is scheduled away from an acute clotting episode and after heparin is stopped, because both distort the result — the provider decides the timing.

When a retest makes sense

A low result is usually confirmed with a repeat test after the acute phase and off heparin. A confirmed deficiency is lifelong and is managed by a specialist.

Who usually reviews this result

A result outside the reference range is usually reviewed by: your GP or primary care doctor; a hematologist for confirmed deficiency. Bring your lab report and any previous results to the appointment.

Names used on lab reports

Your report may use any of these names:
Antithrombin IIIAT IIIAT3Antithrombin activity

Units

Measured in: %

Several markers on your report?

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Frequently asked questions

Is low antithrombin the same as thick blood?

Not exactly — the term describes a faster tendency to clot. Low antithrombin removes one of the main brakes on clotting, so clots form more easily.

Can the level recover?

Acquired deficiency often improves once the cause (illness, protein loss, heparin) resolves. Inherited deficiency is permanent.

Does a low result mean blood thinners for life?

Not automatically. The decision depends on whether clots have occurred and on the overall risk picture — it is made by a hematologist.

Sources

Antithrombin (test directory, reference interval) — Labcorp. Accessed: August 31, 2026.

Antithrombin III