Factor VIII activity in cats and dogs

What it shows

Clotting works like a relay. Platelets form the first plug, and then a chain of clotting factors builds a fibrin mesh that holds it in place. Factor VIII is a link in the intrinsic part of this chain, which the aPTT checks; PT checks the extrinsic part and does not depend on factor VIII. So a prolonged aPTT with a normal PT points to the intrinsic factors, and hemophilia A is the most common inherited disorder in this group.

When factor VIII is missing, bleeding is typically deep: into joints with lameness, into muscles as large swellings (hematomas), into body cavities, or as bruising. A prolonged aPTT alone cannot diagnose hemophilia A — a specific factor VIII assay is needed, and only factor assays tell hemophilia A from hemophilia B (factor IX deficiency).

Higher than reference — what it may be a sign of

A result at or above the lower limit of the interval makes hemophilia A unlikely as the cause of bleeding; the vet then looks at other steps of clotting — platelets, von Willebrand factor, other factors.

Lower than reference

• Hemophilia A — activity of 20% or less is consistent with the diagnosis. One grading is mild 6–20%, moderate 2–5% and severe below 2%; most affected dogs are below 6%. Dogs below 5% often have joint bleeding, hematomas and blood in body cavities; at 5–10% they often do not bleed on their own but bleed longer after injury or surgery.
• Carrier females — usually intermediate activity of about 40–60%, compared with above 60% in normal dogs; screening tests are normal. The factor VIII test may miss some carriers.
• Young animals — under 6 months the immature liver may make less of the clotting factors, so results, especially for carrier testing, are interpreted with caution.
• Sample problems — clots or a wrongly filled citrate tube make the result invalid, and the lab asks for a new sample.

Look at together with

• aPTT and ACT — usually prolonged in affected animals, though ACT may be normal in mild or moderate hemophilia; individual factors usually need to fall below about 30% to prolong aPTT.
• PT — normal with factor VIII deficiency alone.
• von Willebrand factor — normal or higher in hemophilia A, which helps separate it from von Willebrand disease.
• Platelets and hematocrit — platelet problems as another cause, and how much blood has been lost.

Units and conversion

Factor VIII activity is reported in percent (%) of a standard plasma pool from healthy animals of the same species, taken as 100%. Forms may write “factor VIII”, FVIII or FVIII:C (factor VIII coagulant activity). 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 it is valid only for the species the lab’s pool comes from.

Carrier detection is harder than diagnosis. Measuring von Willebrand factor and its ratio to factor VIII can help, but it is not fully reliable and some carriers are still misclassified, so family history is considered together with the lab result. Males can be confirmed clear by testing. The sample is specially prepared: citrate tube filled exactly, plasma separated promptly, then chilled or frozen; hemolysis also suggests an improper draw.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Inherited deficiencies (hemophilia A).
2. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Coagulation assays (PT, APTT).
3. McQuinn E. Coagulation Protein Disorders in Animals (hemophilia A). Merck Veterinary Manual, full review February 2025.
4. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Hemophilia A (clinical topic).
5. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Sampling Instructions.