Prolonged aPTT in a young cat: haemophilia or factor XII deficiency
7-month-old male British Shorthair cat with a large scrotal haematoma and delayed bleeding after castration. It previously had a haematoma at the vaccination site. No access to rodenticides.
| Parameter | Result | Reference |
|---|---|---|
| Platelets | 310,000/µL | 200,000–600,000 |
| PT | 9.8 s | 8.0–12.0 |
| aPTT | 38.0 s | 12.0–22.0 |
Which test best confirms the cause of the bleeding?
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A. Measure von Willebrand factor
Von Willebrand disease is rare in cats and mainly affects primary haemostasis (mucosal bleeding). The pattern of deep haematomas and delayed bleeding with a markedly prolonged aPTT points to an intrinsic pathway factor deficiency.
B. Repeat PT after 5 days of vitamin K1
PT is normal and there is no exposure to anticoagulants. Vitamin K deficiency prolongs PT (factor VII, short half-life) before aPTT.
C. Measure factor XII, because its deficiency is the most common in cats
Factor XII deficiency is common in cats and markedly prolongs aPTT, but causes no clinical bleeding. It would not explain the haematomas; it would be an incidental finding in a healthy cat.
D. Measure factor VIII and IX activity
Correct. A young male with haematomas and delayed bleeding, prolonged aPTT and normal PT suggests haemophilia A (factor VIII deficiency) or B (factor IX deficiency), both X-linked. Haemophilia B has been described in British Shorthairs. Measuring both factors distinguishes them.
lightbulb Key takeaway
Prolonged aPTT with normal PT = intrinsic pathway (factors XII, XI, IX, VIII). If there is bleeding, think haemophilia A or B (males, X-linked); if the cat does not bleed, suspect factor XII deficiency, a laboratory finding without clinical consequence.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.