Cat Coagulation Panel: PT, aPTT and Platelet Ranges

Three numbers answer most bleeding questions in a cat: prothrombin time (PT) 8 to 12 seconds, activated partial thromboplastin time (aPTT) 15 to 25 seconds, and platelets 150 to 400 x10^9/L. Those are textbook intervals for feline plasma and whole blood. Every laboratory sets its own, often 1 to 2 seconds wider or narrower, so the interval printed on the report outranks any figure in a book. The rest of this page is what each test reads, what pushes it out of range, and how 3 numbers become 3 tiers of bleeding risk.

What the 3 numbers watch

PT reads the extrinsic and common pathway: factor VII first, then X, V, II and fibrinogen. aPTT reads the intrinsic and common pathway: XII, XI, IX, VIII, then the same X, V, II step. Platelets are the first responder, sealing a small hole in 1 to 3 minutes. A cat can bleed from failure of any one of the 3 while the other 2 read normal, which is why the panel is read as a set and never as a single flagged line.

PT at 8 to 12 seconds

Anticoagulant rodenticide is the classic cause in cats. Second-generation products block vitamin K epoxide reductase, and because factor VII has a half-life near 6 hours, PT rises within 24 to 48 hours of ingestion, well before any visible bleeding. Liver failure, disseminated intravascular coagulation (DIC) and inherited factor VII deficiency also lengthen PT.

Read PT as a ratio, patient over control from the same run: up to 1.3 is normal, 1.3 to 1.6 is a warning, above 1.6 means bleeding is likely with surgery or trauma. The INR was built for people on warfarin and is not validated in cats; ask for the ratio, not the INR. A single PT of 14 seconds with no bruising is a reason to repeat in 24 hours, not to start treatment.

aPTT at 15 to 25 seconds

aPTT moves with heparin, with hemophilia A or B (factor VIII or IX, rare but real in cats), and with DIC. A short aPTT means nothing clinically. A long one needs a control tube drawn at the same time, because the usual cause is the sample, not the cat: a 3 mL citrate tube filled to 1.5 mL has 2 times the intended citrate and can add 10% to 20% to the result. Hemolysis, lipemia and a partly clotted tube all lengthen aPTT, and a value 3 seconds over the top of the interval in a well cat is usually one of those 4.

Platelets at 150 to 400 x10^9/L

Feline platelets are large, sticky and prone to clumping, so the automated count under-reads more often than it over-reads. A machine count of 80 x10^9/L with clumps on the smear may be 200 x10^9/L by hand. The manual estimate is quick: average platelets per oil-immersion field, multiplied by 15,000 to 20,000, gives the count per microliter. Under 50 x10^9/L, spontaneous bruising becomes possible; under 20 x10^9/L, bleeding can start without trauma.

Test	Typical feline interval	Unit	Action threshold	Common cause of a bad result
PT	8-12	seconds	ratio over 1.6	rodenticide, liver failure, DIC
aPTT	15-25	seconds	ratio over 1.6	underfilled tube, heparin, hemophilia
Platelets	150-400	x10^9/L	under 50	clumping, EDTA artifact
Fibrinogen	100-300	mg/dL	under 100	DIC, liver failure
BMBT	under 4	minutes	over 4	platelet function defect

3 tiers of bleeding risk

Tier 1, low: platelets 100 to 400 x10^9/L, PT and aPTT within 1.3 of control, no bruising. Dental work and elective surgery are reasonable, and no product is needed. Tier 2, moderate: platelets 50 to 100, or a ratio of 1.3 to 1.6, or petechiae over the gums and belly. Surgery is possible but plan vitamin K, plasma and a pressure bandage before the first cut. Tier 3, high: platelets under 50, any ratio over 1.6, active bleeding from 2 or more sites, or melena. No elective procedure; treat the cause, transfuse plasma or platelets where available, and reassess with a repeat panel at 12 to 24 hours.

Sample handling that changes the answer

Use a 3.2% sodium citrate tube, fill it to the line (blood to citrate 9 to 1), invert 3 or 4 times, and centrifuge at 1,500 g for 15 minutes within 1 hour. Plasma keeps 4 hours at room temperature and 2 weeks frozen at minus 20 C for a batched run. Draw before heparin is started, not after; a cat on heparin has an aPTT that reflects the drug, not the disease. In a 4 kg cat, 1.8 to 2.7 mL of blood in a 2.7 mL tube is enough, so ask for a pediatric tube rather than a half-filled adult one.

Signs that mean the number is already late

Pinpoint red spots on the gums or inner ear, or bruises 2 to 3 mm across that were not there yesterday.
Nosebleed lasting over 5 minutes, or blood in the stool that looks like tar.
Buccal mucosal bleeding time over 4 minutes, measured with a spring device on the inside of the lip.
Pale gums with a heart rate over 200 beats per minute after a fall or a fight.

Rodenticide exposure deserves a call even without signs, because PT at 48 hours is the test that decides whether 4 weeks of vitamin K is needed. Photograph the product label and bring it in, since the active ingredient sets the dose. A cat with a prolonged aPTT and a normal platelet count also needs a white cell and red cell review, since marrow disease and immune destruction often travel together; the white cell count guide covers that reading. Breed matters only in a few lines, and the breed quiz is a faster way to check whether a cat is one of them.

5 steps when a result comes back abnormal

1) Ask for the lab interval and the control value, not just the patient number. 2) Repeat the panel on a fresh, properly filled tube before any treatment. 3) Add a platelet estimate by hand and a smear review. 4) If bleeding is active, treat first and confirm later. 5) Recheck at 24 hours for rodenticide and at 12 to 24 hours for DIC, and write the trend, not the single value, into the record.

Terms mentioned in this article

Frequently asked

Is a PT of 14 seconds dangerous in a cat?

Only in context. With a control near 11 seconds the ratio is 1.27, inside the warning band but below the 1.6 line where surgery bleeding becomes likely. Repeat in 24 hours on a properly filled citrate tube, check for rodenticide access, and look at platelets before treating a number.

Why do labs give different aPTT ranges?

Reagents differ in phospholipid composition and activator, and each analyzer adds its own bias, so one lab may print 15 to 25 seconds and another 17 to 30. That is why the report interval and a same-run control matter more than the textbook figure, and why comparing results from 2 labs is unreliable.

Can a cat with normal platelets still bleed?

Yes. Platelet count says nothing about function, and 150 to 400 x10^9/L with a buccal mucosal bleeding time over 4 minutes points to a function defect, often drug related or inherited. von Willebrand disease and aspirin-like drugs both produce a normal count with prolonged bleeding.