Changes in factor VIII activity and von Willebrand factor antigen concentration with age in dogs.

The factor VIII activity of 38 German shepherd puppies, 6-12 weeks old, submitted for diagnosis of haemophilia A was measured. Eight of these puppies had values higher than would be expected for haemophiliacs, but less than the reference range for adult dogs. A further sequential study of 21 puppies (6-26 weeks of age) indicated that the factor VIII activity of puppies is generally less than that of adult dogs until about 14 weeks of age. Changes in the concentration of von Willebrand factor antigen in the puppies were irregular. These variations are probably not sufficient to interfere with accurate diagnosis of haemophilia A in most affected young dogs, but may interfere with the detection of heterozygotes in young bitches.