Prothrombin time (PT, PT %, INR) in cats and dogs

What it shows

Clotting proteins (factors) work as a chain that ends in a fibrin clot. PT starts this chain at factor VII and runs through the common part — factors X, V, II and fibrinogen.

PT is not very sensitive: a factor usually has to fall below about 25–30% of normal activity before PT lengthens. Several mild deficiencies at once, as in liver disease, add up.

A form can show the result in three ways:

• PT, seconds — the clotting time itself, usual in veterinary labs.
• PT %, prothrombin activity (“Quick %”) — the same test turned into a percentage using a curve made from diluted normal plasma taken as 100%. The logic is reversed: the lower the percentage, the longer the clotting time. This scale is still used in some European countries in human-medicine labs.
• INR — a ratio from human medicine, made to compare PT between labs in people taking warfarin-type anticoagulant tablets. The veterinary sources here give no INR reference for cats or dogs, so read it only against the interval on the form.

Higher than reference — what it may be a sign of

• Anticoagulant rodenticide (rat poison): these poisons stop the liver from activating factors II, VII, IX and X. In dogs and cats PT rises first and APTT shortly after, but not at once, so when exposure is uncertain vets may check PT about 48 hours later. This is an emergency; the vet decides what to do.
• Liver failure: most clotting factors are made in the liver. APTT and/or PT is prolonged in 50–75% of dogs with severe liver disease.
• DIC (disseminated intravascular coagulation): factors are used up during a severe underlying illness. DIC is judged from several tests together with that illness, never from PT alone.
• Inherited factor VII deficiency: PT long, APTT normal; usually found by chance and rarely causes spontaneous bleeding.
• The sample itself: an underfilled citrate tube, a difficult draw or very concentrated blood (high hematocrit) can falsely lengthen PT.

Lower than reference

A PT shorter than the reference range has no diagnostic meaning: it does not mean the blood clots “too easily” and is usually a sampling or lab error. A PT inside the range does not exclude a mild deficiency: the test reacts only to large drops. On the percentage (Quick) scale it is the other way round: a low percentage means the same as a long PT in seconds.

Look at together with

• APTT — the pair to PT. Long PT with normal APTT points to factor VII; long APTT with normal PT — to the intrinsic factors; both long — to several factors or the common part of the chain (rodenticide, liver failure, DIC).
• Fibrinogen and thrombin time — very low fibrinogen lengthens all clotting times.
• Platelets and D-dimer — part of the set of tests used when DIC is suspected.

Units and conversion

PT is reported in seconds (s); nothing needs converting. PT % and INR are separate rows: a percentage and a ratio without a unit. They cannot be turned into seconds or into each other without the lab’s calibration data, so the app keeps each on its own line.

Reference range

The reference range is on your lab’s form. PT depends on reagent and analyser; results from different labs are not directly comparable. For illustration: one reference coagulation laboratory lists 11.0–15.5 s for dogs and 15.0–20.0 s for cats, an in-clinic analyser using citrated whole blood gives 11–17 s and 15–22 s, and with some other reagents dog and cat PTs are under 10 s.

Before trusting a long PT, the sample is checked. The citrate tube needs nine parts blood to one part citrate, and underfilling lengthens clotting times. A slow, interrupted draw, clots, foaming or hemolysis spoil the result. Plasma is separated quickly and sent cooled or frozen: clotting factors are unstable.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Coagulation assays.
2. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Sample collection.
3. McQuinn E. Coagulation Protein Disorders in Animals. Merck Veterinary Manual, professional version, updated March 2026.
4. Center SA. Coagulation Tests in Hepatic Disease in Small Animals. Merck Veterinary Manual, professional version, updated November 2024.
5. Tauer D. Anticoagulant Rodenticide Poisoning in Animals. Merck Veterinary Manual, professional version, November 2025.
6. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Assay Reference Intervals.
7. IDEXX — Coag Dx Citrate PT Test, package insert.
8. MSD Manual Professional Edition (human medicine). Excessive Bleeding — PT and INR.
9. Tripodi A, Lippi G, Plebani M. How to report results of prothrombin and activated partial thromboplastin times. Clin Chem Lab Med 2016;54(2):215–222 (human medicine).