Red blood cells are essential for carrying oxygen to the cells of our horses bodies. For optimum health and performance the mass of red blood cells must be also optimized. We measure red blood cell mass by taking a whole blood sample placing a small amount in a long thin tube and centrifuging it until all the cells are squeezed to one side. Then the column of red blood cells is compared to the total column of cells and fluid. The ratio of cell column over the total column is called the packed cell volume (PCV) or hematocrit (Hct).

Anemia is a frequently diagnosed disease in horses however there are some unique features of equine physiology that complicate the accurate diagnosis of anemia. Further complicating correct diagnosis is that there are significant differences in the normal hematocrit of different breeds of horses. The result is anemia is over diagnosed, even by equine veterinarians. Caution should be applied to the diagnosis of anemia in an otherwise healthy horse.

Besides anemia as a cause of serious of decreased performance, there are serious and life threatening diseases where anemia is the main cause of disease symptoms. This article discusses the clinical signs, accurate diagnosis of anemia, and the causes in horses. Links are provided to articles on the specific causes where further treatment is discussed.

The most important point to understand is that the horse has a large splenic reserve of red blood cells. Horses at rest store as much as 30% of their RBC's in the spleen, a large muscular organ on the left side of the abdomen. When the horse is exercised or excited, the spleen contracts releasing its reserve of blood cells into the blood stream. At rest the packed cell volume (the most common measure for anemia) may be in the low twenties and a diagnosis of anemia made when the horse is perfectly normal. Take the same horse and lunge him for five minutes and his PCV will rise. A horse must be exercised briefly, but aggressively just prior to testing for accurate, repeatable results.

RBC's are constantly being produced, age, and when old are removed from the blood and broken down. Some of the components are spared and reused. The life of the average RBC is about 150 days.

Red blood cell measurements are usually taken as either packed cell volume or hemaotcrit which mean the same thing: the percentage of total volume of a centrifuged whole blood sample that is red blood cells:

• Normal values for the hot breeds are 32 to 48%.
• Normal values for ponies and cold breeds are 24 to 44%.

When below these values the horse is considered anemic though you often find healthy horses with a value a few points below the aabove values. Above 48 is most often do to hydration. Other important blood parameters are:

Parameter	Normal Value	Significance
Hemoglobin weight of the oxygen containing molecules in the RBC	10 to 16 gm/dl	When expressed in these units there is usually a numeric ratio of 1 to 3 of the hemaglobin to hematocrit. When higher it suggests hemolysis with free hemaglobin in the blood.
Reticulocytes premature RBCs	0	Unlike other species they do not tend to appear with regenerative anemias.
MCV Volume of a single RBC.	34 to 58 fl	Unlike other species MCV does not typically rise in regenerative anemias. With iron deficiency (rare in horses) however all three values will be low.
MCH Amount of hemeglobin in a single RBC.	13 to 19 pg
MCHC Concentration of Hb in a single RBC.	31 to 37 g/dl
Platelets Necessary for clotting they originate from the same line of progenator cells that the red and white blood cells do in the bone marrow.	100 to 400 x 1000 / uL	Low values may indicate blood loss, disorders of coagulation (coagulopathies), or bone marrow disease.