According to the article Feline Blood Transfusions: A pinker shade of pale by (Barfield and Adamantos, 2010), requirements for a feline blood donor include a healthy, normal body condition, indoor cat with an agreeable temperament for the benefit of easy handling and restraint.
The donor must be 5 kg in minimum as recommended by the authors and should not donate blood more often than once every 2 months. Cats weighing more than 4 kg can be used although fewer side effects should await larger donors. In addition, according to (Helm and Knottenbelt, 2011), ideally, the donor must not be brachycephalic, young to middle-aged (1 to 8 years old), have had no previous pregnancies and female donors should be spayed.