In dogs and cats, increased reticulocytes indicate a regenerative anemia, where the bone marrow is responding to a need for more red blood cells (RBC) by releasing immature RBCs. (Note: horses do not release reticulocytes into circulation so cannot use this parameter to determine if anemia is regenerative vs. nonregenerative.)
To determine the reticulocyte count, blood is stained with new methylene blue which binds the reticular RNA in the immature RBC. Here is an example of reticulocytosis in an anemic cat, the reticulocytes contain bluish-stained dots/clumps of stained RNA.
Immature RBCs are polychromatophilic (i.e., different RBC colors, immature RBC typically appear bluish) on routine Romanowsky-based stains (e.g., Wright’s, DifQuik). Polychromasia suggests a regenerative anemia, but perform a reticulocyte count to confirm.
Reticulocytes can be punctate (mature) or aggregate (immature), based on how the RNA stains. This is relevant in cats, in which punctate retics can persist in circulation for 7-10 days and so do not accurately represent the regenerative response. In cats, only count aggregate retics are counted. In other species, both types can be counted because they have similar half-lives.
