Antithrombin (AT III) in cats and dogs

What it shows

If the clotting chain is the accelerator, antithrombin is one of the main brakes. It blocks most activated clotting factors, above all thrombin and factor Xa. Its action is greatly strengthened by heparin — on vessel walls by heparin-like molecules, in hospital by heparin given as an anticoagulant, which works through antithrombin.

Antithrombin is made in the liver and is a small protein about the size of albumin. So it can be lost together with albumin when protein leaks through the kidneys or gut, and it falls when the liver makes too little.

The lab measures activity, not amount: plasma is mixed with heparin and an excess of thrombin or factor Xa, and a colour reaction shows how much of the enzyme was left unblocked. The less is left, the more antithrombin the sample had.

Higher than reference — what it may be a sign of

A high antithrombin has unknown diagnostic relevance. It has been reported in cats with feline infectious peritonitis, heart disease and hyperthyroidism. The idea that in cats antithrombin rises with inflammation is untested and unproven.

Lower than reference

• Loss with protein — protein-losing kidney disease (nephropathy) and protein-losing gut disease (enteropathy), usually alongside low albumin. Loss into protein-rich effusions or with oozing skin disease is possible but not confirmed.
• Consumption in DIC — antithrombin is used up while clots form, which makes it a sensitive test for disseminated intravascular coagulation in dogs.
• Reduced production — liver failure and abnormal portal blood flow, including portosystemic shunts. In one study of 20 dogs with liver failure, 84% had antithrombin below 75%. Inflammation also lowers production, because antithrombin behaves as a negative acute-phase protein.
• Some medicines — for L-asparaginase, used in cancer care, a fall is supported by test-tube studies.

A low value is a risk marker, not a diagnosis of thrombosis: in protein-losing disease low antithrombin does not always go together with actual clots. The vet weighs it with the rest of the picture.

Look at together with

• Albumin and total protein — low together with antithrombin point to protein loss.
• Urine protein : creatinine (UPC) — shows protein loss through the kidneys.
• D-dimer, fibrinogen, PT, aPTT — with antithrombin they make up a typical DIC panel.
• Platelets — another part of the DIC picture.
• Bile acids and other liver tests — when liver failure or a shunt is suspected.

Units and conversion

Antithrombin is reported in percent (%): the activity in the sample compared with a standard pool of plasma from healthy animals of the same species, taken as 100%. Forms write it as AT, AT III or ATIII — the same test. There is nothing to convert; in the app each row keeps the unit from the form.

Reference range

The reference range is on your lab’s form, and it differs between species. The Cornell coagulation laboratory, for example, lists 65–145% for dogs and 75–100% for cats. In dogs antithrombin deficiency is generally taken as activity below 50%, and it is that level that is linked to a higher risk of thrombosis.

The sample is collected as for any clotting test — how, see the article on PT.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Inhibitors: Antithrombin.
2. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. DIC and Thrombosis (clinical topic).
3. Pierce KV, Slack J. Thrombosis, Embolism, Aneurysm, and Dissection in Animals. Merck Veterinary Manual, August 2025.
4. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Assay Reference Intervals.
5. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Sampling Instructions.