Upper panel: Polychromatophilic RBCs in regenerative anemia in a dog. Lower panel: Reticulocytes stained with NMB from the same dog. Canine reticulocytes: In dog blood, any red cell that contains RNA is called a reticulocyte. The reference range for non-anemic dogs is 0 to 1.5% reticulocytes. A reticulocyte count >1.5% is reticulocytosis regardless of the HCT. If an anemic dog does not have reticulocytosis, the anemia is either non-regenerative or the onset has been too recent (within 2-3 days) for a regenerative response to be evident in blood. If an anemic dog has reticulocytosis, then one must decide whether the increased percentage of reticulocytes indicates an actual increase in erythropoiesis. To assess the adequacy of the regenerative response in relation to the severity of the anemia, several formulas can be used: Absolute reticulocyte count This takes into account the severity of the anemia and is the easiest formula to use. It is calculated as follows: Absolute reticulocytes = reticulocyte % x RBC count

Degree of Regeneration	Absolute reticulocytes (/uL)
Normal	60-80,000
Mild	150,000
Moderate	300,000
Marked	500,000

Corrected reticulocyte percentage (CRP) This compensates for the degree of anemia, but it makes assumptions as to what the normal hematocrit of the patient is (45% for a dog and 35% for a cat). This can be totally inappropriate, e.g. a 45% hematocrit is not normal for Greyhounds (and would actually be consistent with anemia). The CRP is calculated as follows: CRP = reticulocyte % x (patient's HCT / normal HCT) Reference ranges for CRP are < 1% in the dog and < 0.4% in the cat. Red cell production index (RPI) Calculation of the RPI takes into account the degree of anemia and the rate of erythrocyte maturation in the circulation. This assumes that the predicted lifespan of reticulocytes is similar in human beings and dogs. Although this may not be true, the RPI does seem to work in the dog only. The RPI is calculated as follows: RPI = CRP/reticulocyte lifespan (days) The reticulocyte lifespan is illustrated in the following table:

HCT	Lifespan
45	1.0
35	1.5
25	2.0
15	2.5

An RPI of 1.0 reflects a normal rate of red cell production. Values greater than 1.0 indicate increased production in response to anemia.