APTT (activated partial thromboplastin time) in cats and dogs
Activated partial thromboplastin time (APTT, aPTT, sometimes simply PTT) is the number of seconds citrated plasma needs to clot after an activator, phospholipid and calcium are added. It reflects the clotting factors of the intrinsic and common pathways. Only a prolonged APTT matters; a short one is usually a sampling artefact.
What it shows
Clotting factors work as a chain that ends in a fibrin clot. APTT starts the chain from the “contact” side — factors XII, XI, IX and VIII — and runs through the common part: factors X, V, II and fibrinogen. Its pair, prothrombin time (PT), starts from factor VII instead; together the two tests cover the whole chain.
The test is not very sensitive: a single factor usually has to fall below about 30% of its normal activity before APTT gets longer. The cruder in-clinic version, the activated clotting time, needs a drop below about 10%. Forms may write the test as aPTT, APTT or PTT — it is the same measurement.
Higher than reference — what it may be a sign of
- Inherited factor deficiencies: hemophilia A (factor VIII) is the most common inherited clotting-factor deficiency in dogs and cats; hemophilia B (factor IX) is rarer. Both lengthen APTT while PT stays normal. Factor XI and XII deficiencies do the same; factor XII deficiency causes no bleeding.
- Anticoagulant rodenticide (rat poison): APTT rises after PT; in dogs it can take about 48 hours. This is an emergency — the vet decides what to do.
- Severe liver disease: the liver makes most clotting factors; APTT and/or PT is prolonged in 50–75% of dogs with severe liver disease.
- DIC (disseminated intravascular coagulation): in dogs APTT is more sensitive to it than PT. DIC is judged from several tests together with the underlying illness, never from one number.
- Heparin — this anticoagulant medicine lengthens APTT, which is why APTT is used to watch its effect.
- The sample itself: an underfilled citrate tube, very concentrated blood (high hematocrit) that leaves too much citrate in the plasma, or a difficult draw.
Lower than reference
A short APTT has no diagnostic meaning and does not show that the blood clots “too easily”. It usually comes from the sample: a slow or difficult draw activates clotting in the tube and can shorten APTT or make a truly long one look normal. An APTT inside the range does not rule out a mild deficiency, because the test changes only after a large drop in factor activity.
Look at together with
- PT — the pair. Long APTT with normal PT points to the intrinsic factors (hemophilia, factor XI or XII); both long — to several factors or the common part (rat poison, liver failure, DIC).
- Fibrinogen and thrombin time — very low fibrinogen lengthens all clotting times.
- Platelets and D-dimer — the usual companions when DIC is suspected.
- Von Willebrand factor — sometimes added to a PT and APTT panel when an animal bleeds for no clear reason.
Units and conversion
APTT is reported in seconds (s); nothing needs converting. The app keeps the value as printed on the form.
Reference range
The reference range is on your lab’s form. APTT depends on the activator, the reagent and the analyser, so results from different labs are not directly comparable — compare repeat tests from the same lab. For illustration: one reference coagulation laboratory lists 11.0–17.5 s for dogs and 13.3–20.2 s for cats, while another teaching source gives about 15–20 s for dogs. People have much longer times, about 25–45 s.
Before trusting a long APTT, the sample is checked. The citrate tube needs nine parts blood to one part citrate; underfilling lengthens clotting times. The needle should enter the vein cleanly, and if the blood stops flowing a new draw is needed. The plasma is separated as soon as possible and sent cooled within about a day or frozen, because clotting factors are unstable.
Cats and dogs
Factor XII deficiency is common in cats and does not cause bleeding, so a long APTT in a cat without signs is likely explained by it and found by chance. Hemophilia B has been reported in Siamese-cross and British Shorthair cats. In DIC, PT is more often prolonged in cats than APTT is.
Hemophilia A is X-linked; affected dogs with less than about 5% of normal factor VIII activity often bleed into joints and body cavities, while those with 5–10% usually bleed only after injury or surgery. Factor XI deficiency is reported in Kerry Blue Terriers and Weimaraners, and prekallikrein deficiency, which also lengthens APTT, in Poodles. Some breeds are screened before surgery for this reason.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Coagulation assays.
- eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Sample collection.
- McQuinn E. Coagulation Protein Disorders in Animals. Merck Veterinary Manual, professional version, updated March 2026.
- McQuinn E. Overview of Hemostatic Disorders in Animals. Merck Veterinary Manual, professional version, updated March 2026.
- Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Assay Reference Intervals.
- VCA Animal Hospitals — Coagulation Tests.
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.