Anemia is a condition where the red blood cell count is lower than normal values. Red blood cells are measured as hematocrit (HCT) or packed cell volume (PCV.) These essentially measure the same thing: the percentage of blood volume made up by red blood cells. Testing labs have slightly differing normal ranges for dogs, but in general normal is 38-59% HCT or PCV. Sight-hounds in particular have very high normal values. Anemia can be mild, moderate or severe.
Anemia can be caused by a number of things including: excessive bleeding (whether internal or external), red blood cell destruction (as in AIHA and IMHA), either reduced production of RBC or complete bone marrow failure, and some diseases like kidney disease. When a dog is anemic, it is important for the vet determine what the cause is so they can apply treatments.