Pearls
- Look at the Hct first and see if it is high, low, or normal with respect to the RI.
- If the Hct is low, the Hgb and RBC count will generally also be low, in direct relationship to the Hct. As a general rule of thumb, if you multiply the Hgb by .0033 (or just x 3 and move the decimal point), you approximate the Hct.
- Evaluate the RBC indices for: size (MCV), hemoglobin content (MCH), and hemoglobin concentration (MCHC).
- A low Hct generally indicates anemia (zealous fluid therapy can dilute the blood causing a drop in Hct without a true anemia being present, so history, as always, is important).
- The next step in evaluating anemia is to determine if it is regenerative (responsive bone marrow) or nonregenerative (nonresponsive bone marrow). This greatly focuses the search for potential causes.
- Reticulocyte numbers should be elevated if the bone marrow is responsive. The reticulocyte count is performed with a vital stain (NMB) which precipitates RNA present in immature RBCs. The reticulocyte production index (RPI) is calculated in dogs only. The reticulocyte count (% and absolute numbers) should correlate with the degree of polychromasia (large, bluish, immature RBCs seen with Romanowsky stains) noted in the RBC morphology section of the report.
- Note: Horses are an exception in that they rarely release polychromatophilic RBCs/reticulocytes in response to anemia. To evaluate anemia in the horse, look at the MCV; often it will be slightly elevated if the marrow is responsive (releasing larger cells). Anisocytosis and RDW may be increased. We also follow the CBC over time, to watch for a rising Hct (also indicative of marrow response). If necessary, bone marrow can be examined to assess erythroid activity in the horse.
- The degree of regeneration is also important. If an animal is severely anemic, but only has a mild degree of regeneration, this is an inappropriate response. Sometimes this can be explained by the time frame. For example, if an animal hemorrhages acutely from being hit by a car, a regenerative response will not be seen for 2-4 days and will not peak for about 5-6 days after that, with some species variability. Repeating the CBC is very useful in these circumstances.
- Hallmarks of regenerative anemia, in decreasing order of importance: increased polychromasia, increased reticulocytosis, increased RPI (dogs only), increased MCV, macrocytosis, increased anisocytosis, presence of nRBCs, increased RDW. Nucleated RBCs alone, without polychromasia, do not indicate regeneration. Note, all of the above may not be present in every situation. The cause of the anemia, duration, species, and other factors will determine the features that are present. Polychromasia and reticulocytosis are the most important criteria of RBC regeneration in species other than the horse.
- Anemias with low MCV +/- low MCH +/- low MCHC, with or without signs of regeneration, should signal possible iron deficiency and trigger a search for causes of chronic external blood loss in the animal, or dietary iron lack in a neonatal animal. A low MCV can also be seen in dogs with portosystemic shunts, with or without accompanying anemia, and in nonanemic dogs of certain breeds.
- Red cell morphology can provide clues as to potential causes of anemia – e.g. Heinz bodies, eccentrocytes, spherocytes, acanthocytes, echinocytes III, keratocytes and schizocytes all have significance which may relate to erythrogram changes (see Table 1.1). There may be classic morphologic changes that could lead one directly to a diagnosis, for instance, the presence of spherocytes, agglutination, possibly ghost cells, and increased polychromasia and reticulocytosis would be highly supportive of a diagnosis of IMHA.
- The RDW is an index of the degree of anisocytosis (variability in RBC size) and should be useful in assessing anemias. Large, young cells mixed with older, smaller cells should increase the RDW; the RDW should be particularly high with immune-mediated hemolytic anemia where there are often immature cells mixed with cells that are small due to losing pieces of their membranes (spherocytes). Unfortunately, the RDW does not always reflect the degree of anisocytosis seen on peripheral blood examination, which emphasizes the importance of smear evaluation.
- Don’t forget to look at platelets. If an anemic animal has no platelets, you may have identified the cause of the anemia without going further. Then you shift to determining why there are no platelets (thrombocytopenia).
- If the Hct is elevated, determine if the change is relative (due to dehydration or splenic contraction) or absolute. If the erythrocytosis is absolute, then explore potential causes (appropriate and inappropriate) of increased erythrocyte production.
definition
Increased number of erythrocytes with increased volume that usually corresponds with increased MCV; may be seen in regenerative anemias and can be normal in Poodle dogs.
Erythrocyte with little central pallor and several irregular membrane projections.
Erythrocyte with even projections all around its membrane; often an artifact but can be a pathologic change. Echinocytes are classified as types 1 and 3 depending on the length and sharpness of the projections (spicules).
Crescent-shaped cells that are formed from mechanical shearing (usually due to fibrin strand deposition) of the red cell.
An erythrocyte fragment created by shearing trauma to erythrocytes within the vascular system; often the fragment which has been sheared off to form a keratocyte.
Clumping of erythrocytes due to antibody interactions. Must be differentiated from rouleaux formation.
Anemia due to destruction of RBCs that is caused by antibodies produced by the animal’s own immune system. Usually highly regenerative, with presence of spherocytes and agglu
Decrease in the number of circulating platelets below the reference interval.
