von Willebrand factor antigen (vWF:Ag) in cats and dogs

What it shows

Platelets need a “glue” to hold on at a site of injury, and vWF is that glue. When there is too little of it, or it is built wrongly, the first plug forms poorly. Typical bleeding comes from mucous membranes — nose, gums, urinary, reproductive and intestinal tracts — or after surgery, tail docking, dewclaw removal or teething; pinpoint skin bleeds (petechiae) are rare and point to low platelets first. Platelet count, PT, aPTT and fibrinogen are usually normal.

The test is an immunoassay, most often ELISA: antibodies to vWF show how much protein is present compared with a species-specific plasma pool set at 100%. It measures the amount, not the structure or function. For type 2 disease, where large vWF molecules are missing, specialised labs use multimer analysis, a collagen-binding activity test or platelet aggregation tests.

Higher than reference — what it may be a sign of

A value above the interval is not a sign of disease in itself. Its level fluctuates, and more so during heat, pregnancy and systemic illness, especially liver or inflammatory disease — so the result is most informative in a healthy animal that is not in heat or pregnant.

Lower than reference

• Type 1 von Willebrand disease — less vWF of normal structure, bleeding of variable severity. It is the most common type; breeds listed include the Doberman Pinscher, German Shepherd, Golden Retriever, Dachshund and Poodle, among others.
• Type 2 — low vWF of abnormal structure, with severe bleeding; described in German Shorthaired and German Wirehaired Pointers.
• Type 3 — vWF almost or completely absent, with severe bleeding and a very long bleeding time. Familial cases occur in the Chesapeake Bay Retriever, Dutch Kooiker, Scottish Terrier and Shetland Sheepdog.

A low result is not the same as a bleeding animal. Many dogs with the trait never bleed: in Airedale Terriers bleeding is rare despite a high prevalence, while affected Dobermans bleed quite commonly. Infections, heat or pregnancy, and steroid or thyroid disorders can make bleeding worse.

Look at together with

• Platelet count — the first thing to check in a bleeding animal; in vWD it is usually normal.
• PT and aPTT — usually normal; some dogs have less factor VIII, which can prolong aPTT and ACT.
• Bleeding time (BMBT) — an in-clinic test of the same step; prolonged with moderate to severe deficiency, but not a replacement for vWF:Ag.
• Hematocrit — how much blood has been lost.

Units and conversion

vWF:Ag is reported in percent (%) of a standard pool of plasma from healthy animals of the same species, taken as 100%. Forms may write vWF:Ag, vWf:Ag or “von Willebrand factor antigen”. There is nothing to convert; in the app each row keeps the unit from the form.

Reference range

The reference range is on your lab’s form, and many labs add an interpretation. As an example, the Cornell coagulation laboratory grades canine results like this: 70–180% normal, considered clear of the trait; 50–69% borderline, where carrier or clear status cannot be decided; 0–49% abnormal, considered carriers, some with a bleeding tendency. “Carrier” here is the lab’s label for a result in the abnormal range. Most dogs that bleed because of vWD have values below 35%, and dogs with type 3 have 1% or less.

A borderline result may be clarified with a second test or with DNA testing; for several breeds the causative mutations are known. Clots or hemolysis in the sample give inaccurate results, so the draw and handling matter: citrated plasma is separated and sent cold or frozen. Puppies can be tested from 6–8 weeks of age.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — von Willebrand factor.
2. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — von Willebrand disease.
3. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Canine von Willebrand Disease (clinical topic).
4. McQuinn E. Platelet Disorders in Animals (von Willebrand disease). Merck Veterinary Manual, full review February 2025.
5. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Submission FAQs.
6. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Sampling Instructions.