We’ve talked many times about anemia and its causes. If you remember, anemia is the term for a low red blood cell count. Red blood cells carry oxygen to all of the tissues in our body, so it spells trouble when there aren’t enough of them to do an adequate job. But, have you ever thought about the opposite problem? Can a dog or cat have TOO MANY red blood cells? Surprisingly, the answer is yes.
One way to measure the amount of red blood cells in the blood is to run a simple test called a hematocrit, which is also known as a packed cell volume (PCV). A tiny amount of your pet’s blood is placed inside a thin glass tube and spun down in a centrifuge. The spinning separates your pet’s whole blood into three layers: plasma, white blood cells and platelets, and red blood cells. Once separated, the amount of red blood cells is measured as a total percentage of the entire tube of blood. A normal hematocrit for a dog falls between 35% and 55%.
Polycythemia is the medical term for an abnormally high red blood cell count. Pets with polycythemia have a hematocrit over 55%. However, it is important to note that some breeds, like Greyhounds and other sight hounds, have hematocrits that normally run a little higher. In these breeds, a hematocrit over 65% is considered abnormally high. When red blood cell levels are abnormally high, the blood itself becomes more thick and viscous, which makes your pet feel sluggish and weak and can eventually lead to cardiac arrest or seizures.
So, what causes polycythemia? Most of the time, dehydration is to blame. This is called relative polycythemia. When pets are dehydrated, the plasma portion of the hematocrit is smaller than normal, and the red cell portion makes up the deficit. This raises the hematocrit level up above the normal mark. Anything that causes dehydration can cause a pet to have relative polycythemia, and it is a common laboratory finding in sick patients. Once fluid therapy restores adequate hydration, the hematocrit will fall back into the normal range.
Absolute polycythemia occurs when the hematocrit is above normal for more serious reasons. Polycythemia of this nature can be primary or secondary, depending on whether the hormone erythropoietin (EPO) is involved or not. EPO is made in the kidneys, and it controls the production of red blood cells.
Primary absolute polycythemia occurs independently of EPO, and is termed polycythemia vera, or true polycythemia. This is due to disease arising from the bone marrow, where red blood cells are made.
Secondary absolute polycythemia occurs because of red blood cell production driven by EPO. Sometimes, this occurs for appropriate reasons, like in the presence of cardiac disease or chronic obstructive pulmonary disease, or if you and your pet are visiting a high altitude area. But sometimes, EPO inappropriately stimulates the red blood cell production, as is the case of some kidney tumors.
Whatever the cause, polycythemia is easily found by running a hematocrit. Once a high red blood cell level is found, your veterinarian will search for the underlying cause. If polycythemia is relative, IV fluids will be administered. Absolute polycythemia requires a different approach: phlebotomy, or the removal of blood. Think of it as your pet’s routine blood donation—your vet will pull a specific amount of blood from your pet so that the hematocrit falls to a normal level. This technique is reserved for pets who have clinical signs or those with hematocrits over 75%. In addition to phlebotomy, patients with polycythemia vera may be put on a medication to slow red blood cell production.
Absolute polycythemia is not common—relative polycythemia is seen much more often. Luckily, there is a quick fix for that!