Reticulocyte Counts - Important for Both Anemic and Non-anemic Dogs

BACKGROUND

Reticulocytes are non-nucleated immature erythrocytes representing the stage of maturation immediately prior to the mature erythrocyte. In most mammalian species, the nucleus from the previous stage of maturation, the metarubricyte or nucleated red blood cell (nRBC) as it is most commonly named when found in the peripheral blood, is spontaneously extruded during the maturation process. These reticulocytes take approximately 2-3 days to complete the process of hemoglobin synthesis and removal of cytoplasmic RNA to reach the mature erythrocyte stage, which will stay in circulation for 80-120 days depending on the species of animal. The vast majority of this maturation takes place within the hematopoietic tissues with the bone marrow playing the most prominent role; however, maturing reticulocytes often leave the marrow and continue this maturation process within peripheral hematopoietic tissues and the spleen plays the most prominent role in this extramedullary maturation. In addition, the complete process of red blood cell as well as white blood cell and platelet production and maturation in these extramedullary sites.

Historically, we only have thought about evaluating peripheral blood reticulocyte counts during periods of anemia to help characterize an anemia as regenerative (when reticulocytosis is present) or non-regenerative (when reticulocytosis is absent). However, reticulocytosis can be seen under several different conditions including any time there is a demand on the hematopoietic tissue to produce more erythrocytes than normal as well as transiently under certain physiologic responses. The more appropriate interpretation of a reticulocytosis in most cases is to identify it as evidence of a hematopoietic tissue response to a peripheral demand or stimulus. The same mechanisms that can result in anemia can be seen in cases without anemia if the hematopoietic tissue has the ability to respond and the degree of impact (blood loss or hemolytic disease) is relatively mild.