Thrombin time (TT) in cats and dogs
Thrombin time (TT), also called thrombin clot time (TCT), measures how fast plasma clots once thrombin — the last enzyme of the clotting chain — is added. It skips every earlier step and tests only the final one: fibrinogen turning into fibrin threads. Only a prolonged result matters, and it is always read together with PT and aPTT.
What it shows
Clotting works like a relay: the extrinsic and intrinsic pathways pass the baton to the common pathway, which ends with thrombin cutting fibrinogen into fibrin. In the TT test the lab adds a standard amount of thrombin straight to citrated plasma and times the clot. All earlier runners are bypassed, so the result depends on fibrinogen — how much there is and whether it works — and on anything that blocks thrombin.
TT reacts to low fibrinogen more sensitively than aPTT or PT. A modified version of the same test (the Clauss method) is one way labs measure clottable fibrinogen: a long clot time means little working fibrinogen.
Higher than reference — what it may be a sign of
- Low fibrinogen — when the liver cannot make enough (liver failure) or when it is used up in disseminated intravascular coagulation (DIC). In dogs with DIC, though, fibrinogen is more often normal or even high, because it is also an inflammation protein; it is low in fewer than 30% of cases.
- Fibrinogen that does not work properly (dysfibrinogenemia). The inherited form is uncommon. An acquired form comes from substances that disturb fibrin formation: high levels of fibrin(ogen) degradation products, as in DIC, or abnormal antibodies (paraproteins) made by tumours of plasma cells (multiple myeloma) or B cells (lymphoma, chronic lymphocytic leukaemia).
- Heparin — either given in hospital or contaminating the sample, for example from a catheter flushed with heparin. Heparin blocks thrombin. Low-molecular-weight heparin acts mainly on factor Xa and does not prolong clotting times.
- Instrument artefact — analysers set up for human plasma may miss the very fast clot in dog and cat plasma and report a falsely long time.
Lower than reference
A short thrombin time has no meaning of its own — only prolongation is interpreted. A normal TT does not rule out a clotting problem either: it says nothing about the steps before thrombin, and that is what PT and aPTT check.
Look at together with
- PT and aPTT — together with TT they make the basic coagulation panel. Long TT with long PT and aPTT points to fibrinogen or heparin; normal TT with long PT and aPTT points to earlier factors.
- Fibrinogen — tells whether a long TT is about the amount of fibrinogen or its function.
- D-dimer — clot breakdown; high together with a long TT fits the picture of DIC, which the vet confirms with other findings.
- Globulin — a hint of paraproteins.
Units and conversion
Thrombin time is reported in seconds (s; с on Russian-language forms); there is nothing to convert. The same test can appear as TT, TCT or “thrombin clot time”. 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. It depends on the thrombin reagent and on the instrument. As an example of scale, the Cornell coagulation laboratory, which uses bovine thrombin, lists about 4–9 seconds for dogs and 4–8 seconds for cats; another lab may print a different interval.
The result is only as good as the tube. Coagulation tests use sodium citrate (3.2% or 3.8%) and nothing else — not EDTA, heparin or serum. The tube is filled completely, which gives one part citrate to nine parts blood; an underfilled tube means too much citrate and falsely long times, so the blood is drawn again. Clots or hemolysis in the sample signal a difficult draw and are also reasons to repeat it, unless the hemolysis comes from the animal’s own disease. Catheters are flushed with plain saline, not heparin.
Cats and dogs
In cats the test works the same way and the lab uses its own feline interval. A long TT in a cat is read next to PT, aPTT and fibrinogen; after a hard blood draw or with an underfilled tube the vet may first ask for a new sample.
In dogs a normal TT does not exclude DIC, because fibrinogen often stays normal or high. Inherited fibrinogen defects are uncommon; in one described dysfibrinogenemia ACT (activated clotting time), aPTT, PT and TT were all prolonged. Puppies with an inherited shortage of prothrombin (factor II) have a normal TT while ACT, aPTT and PT are prolonged — TT tests only the last step.
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 (ACT, PT, APTT, TCT).
- eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Fibrinogen.
- McQuinn E. Coagulation Protein Disorders in Animals. Merck Veterinary Manual, professional version, updated March 2026.
- Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Sampling Instructions.
- Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Assay Reference Intervals.
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.