Activated clotting time (ACT) in cats and dogs

What it shows

Think of ACT as a rough, fast version of the aPTT. The tube contains an activator — kaolin, or a mix of celite, kaolin and glass beads (an older diatomaceous-earth tube is no longer available). The tube is kept at 37 °C in a heating block (the armpit can serve as one) and gently tilted at set intervals until a clot is seen.

Unlike laboratory aPTT, ACT relies on the animal’s own platelets and calcium to support the reaction. It becomes prolonged only when a single factor falls below about 10% of its activity, whereas aPTT reacts at below about 30%, depending on the method. So ACT is enough to show a severe factor shortage, but a normal or mildly long ACT does not exclude a milder one. Reading the end point is also somewhat subjective.

Cartridge analysers that measure ACT by electrochemical sensing detect thrombin formation rather than the clot itself; these have not been studied in animals.

Higher than reference — what it may be a sign of

• Inherited factor deficiencies of the intrinsic or common pathway — hemophilia A (factor VIII) and hemophilia B (factor IX) prolong ACT and aPTT; so do deficiencies of factor XI or prekallikrein. With factor X, ACT, aPTT and PT are usually prolonged once activity is below 30%.
• Anticoagulant rodenticide poisoning — aPTT, PT and ACT are usually prolonged. This is an emergency, and the vet decides what to do next.
• von Willebrand disease — some affected animals also have less factor VIII, which can prolong aPTT and ACT.
• Heparin — ACT is one of the clotting tests used in hospital to follow the effect of unfractionated heparin. It is not useful for low-molecular-weight heparin.
• Very low platelet count — below 10,000/µL it may mildly prolong ACT (by 10–20 seconds with the diatomaceous-earth tube); for other activators the effect is unknown. Mild increases are therefore read with caution.

Lower than reference

A short ACT has no clinical meaning — only prolongation is interpreted. If an animal is bleeding and ACT is normal or only mildly long while platelets are normal, the vet usually sends blood for the more sensitive aPTT and PT.

Look at together with

• aPTT — the same pathways, measured more sensitively in the lab.
• PT — the extrinsic pathway; long ACT with long PT points to the common pathway or to rodenticide poisoning.
• Platelets — a very low count can lengthen ACT on its own.
• Bleeding time (BMBT) — checks platelets and von Willebrand factor, which ACT barely sees.
• Hematocrit — how much blood has been lost.

Units and conversion

ACT is reported in seconds (s); there is nothing to convert. In the app each row keeps the unit from the form, and the chart shows all results on one scale.

Reference range

The reference range is on your lab’s form — for ACT that usually means the clinic’s own interval for the tube it uses. Intervals depend on the activator and cannot be carried over from one tube type to another. With one celite–kaolin–glass-bead tube, a published study found 55–80 seconds in dogs and 55–85 seconds in cats; these numbers do not apply to kaolin tubes. Home-made tubes raise doubts about whether each one holds the same amount of activator.

Because blood goes into the tube without anticoagulant, the test is done right away, and the tube is kept warm throughout. A difficult draw can itself trigger clotting, so the result is read together with how the sample was collected. Only a clearly prolonged ACT matters; a mild increase is a reason for further tests rather than a conclusion.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Coagulation assays (ACT, PT, APTT, TCT).
2. McQuinn E. Coagulation Protein Disorders in Animals. Merck Veterinary Manual, professional version, updated March 2026.
3. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Heparin Anticoagulant Therapy (clinical topic).