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Antithrombin III (Activity) Converter
Antithrombin III (AT III), simply called Antithrombin, is a small protein molecule that inactivates several enzymes of the coagulation system. It is a serine protease inhibitor (serpin) produced by the liver. Its principal physiological role is the inhibition of Thrombin (Factor IIa) and Factor Xa, thereby serving as a natural anticoagulant to prevent excessive clotting.
The activity of Antithrombin is dramatically enhanced (by 1000-fold) in the presence of Heparin. Clinically, AT III activity assays are used to screen for hereditary or acquired thrombophilia (predisposition to blood clots) and to investigate heparin resistance (where heparin fails to thin the blood due to lack of AT III). Unlike antigen tests (which measure mass), activity assays measure the functional capability of the protein. Results are typically reported as a percentage of normal activity or in International Units (U/mL).
SI / Scientific
RecommendedInternational Units per volume (1 U/mL = 1 kIU/L).
Clinical Units
Clinical Context
Antithrombin Deficiency increases the risk of Venous Thromboembolism (VTE) significantly. Deficiencies can be Type I (low quantity and low activity) or Type II (normal quantity but dysfunctional protein). Activity assays detect both types.
Normal reference ranges are generally 80% – 120% (0.80 – 1.20 U/mL). Acquired deficiency is common in DIC, liver disease (reduced synthesis), and nephrotic syndrome (urinary loss). Levels must be interpreted with caution during acute thrombotic events or heparin therapy, as consumption can temporarily lower AT III levels.
Clinical References
How to Use This Calculator
Follow these steps to get accurate clinical conversions instantly.
Enter Values
Input your laboratory results into the corresponding unit field. The most common unit for AT III is % Activity.
Automatic Conversion
The calculator works in real-time. As you type in the percentage, the U/mL and Fraction units are instantly updated.
Verify Units
Check the SI Units section if your institution uses International Units (U/mL) instead of percentage.
Reset
Use the Clear button to reset all fields before starting a new calculation.
Frequently Asked Questions & Tips
100% activity is defined as the activity found in normal pooled human plasma, which corresponds to 1 U/mL.
This calculator is for Activity (Functional) assays. Antigen (Mass) assays exist but use different reference ranges and units.