Why Is My Dog or Cat's Blood Thrombocyte/Platelet

Count Low or High ?













Ron Hines DVM PhD

To see what normal blood and urine values are, go here

For an explanation of causes of most abnormal blood and urine tests, go here

To see how tests are grouped, go here

Your Pet's Blood Platelet/Thrombocyte Count aka PLT, Thrombos - Estimated Number or Actual Count

Thrombocyte are also called blood platelets. They circulate in large numbers in your dog or cat's blood, where they are essential to the normal blood clotting process. Thrombocytes are the only cells in your pet's blood stream that lack a nucleus. They are no more than cytoplasm fragments of the megakaryocytes that dwell in your pet's bone marrow.

You veterinarian might order an estimate of their number if he/she is suspicious that you pet is bleeding excessively due to a faulty clotting mechanism. Perhaps, you and your veterinarian are faced with unexplained anemia in your dog or cat. Or, perhaps, your pet bled excessively after a surgical procedure, developed a nose bleed (epstaxis) or passed blood in its stool or blood in its urine. (ref) A thrombocyte count is included in most standard CBC/WBC reports.



Because the clotting of blood is such a complex process (the coagulation cascade) with many different stages and the interplay of many chemicals, mediators and cells; a series of tests is often required to determine exactly what is wrong. Those other tests are prothrombin time (PT), partial thromboplastin time (PTT), fibrinogen level, the d-dmer test, thrombin clot time TCT and, perhaps, some liver function tests (ALT, bile acids).You pet’s blood platelets (thrombocytes) are essential for blood to clot when it needs to. Along with another blood ingredients (fibrinogen/fibrin), they seal the leaks that naturally occur in your pet’s small blood vessels (capillaries) and those that occur due to trauma.

Since platelets only live a week or so, they are continually produced by the stem cells (megakaryocytes) that I mentioned earlier. If problems occur that interrupt that process or if a rogue antibody is produced that destroy them, it will not be long before your pet’s blood will no longer clot normally. A similar lack of functional platelets occurs in certain forms of shock in which platelets form clumps while still in the cat or dog' s blood stream (DIC). The primary sign of a platelet deficiency of either type is bleeding that will not stop. The laboratories that veterinarians depend on determine the number of platelets in your pet’s blood determined in two different ways and often check one method using the other when the results are unexpected.They can estimate your pet’s platelet numbers by examining a stained glass slide (“smear”) under the microscope that was prepared from a drop of your pet’s blood , and just noted on the pet’s lab report that the number was “adequate” “increased” or “low”. Because the thrombocytes of cats “clump” (stick together) quite easily, these estimates tend to be less accurate in felines.The platelets can also be counted in the same manner as your pet’s white blood cells when a complete blood cell count (CBC/WBC) is performed. Laboratories do this either with an automated cell counter or manually using older etched glass grids (hemocytometer/unopette systems). When the automated systems give unusual results, it is best to confirm them with the older methods. (occasionally, larger than normal blood platelets or clumping will throw off the automated analyzers). Bleeding that occurs when platelet counts are too low can occur just about anywhere in or on the pet's body. It can be an unexplained bruises (ecchymoses), pinpoint bloodspots (petechia) on the gums, deep within the eye and affecting vision (retinal hemorrhage), tarry rust-colored stools (melena) or rectal bleeding(hematochezia) . Sometimes your pet’s urine is noticed to be pinkish in color (hematuria) or hemorrhage surrounding the heart (hemopericardium), lungs (hemothorax) or in the abdomen causes labored breathing and distress.

Reasons Why Your Pet’s Platelet Count Might Be Low thrombocytopenia :

Shock, severe trauma, anaphylactic allergic reactions, bacterial toxins (enterotoxemia, consumptive thrombocytopenia), (all can lead to DIC) all use up blood platelets and lead to low counts. Sudden blood loss or red blood cell destruction (acute hemolytic anemias) can also be responsible for thrombocytopenia. Autoimmune diseases, when the pet’s body produces misdirected antibody against its own blood platelets can cause thrombocytopenia (immune-mediated thrombocytopenia, ITP, IMT, lupus, SLE). Platelet counts are also often low in advanced kidney failure. Medications including estrogens, griseofulvin for skin fungal infection (ringworm), trimethoprim/sulfas antibiotics, albendaole wormer can all decrease thrombocyte numbers and can anticancer drugs such as cyclophosphamide. Ehrlichia, babesia and anaplasmosis infection, passed to dogs by ticks, can cause low thrombocyte counts. (ref) Tumors that occur in the pet's bone marrow can crowd out the megakaryocytes that give rise to thrombocytes (these are various leukemias). Infrequently, low platelet numbers and spontaneous bleeding also occur suddenly with no discoverable reason (idiopathic thromboytopenic purpura). There are also rare cases where platelet numbers are adequate but they just don’t seem to work properly (abnormal platelet function, thrombopathia). Automated platelets counts can falsely appear low when platelets clump. That is usually due to too slow a blood flow into the collection needle or failure to mix the tube well just after it was collected. Often platelet counts have to be remarkably low before excessive bleeding occurs in your pet. That is because the body has multiple fail-safe methods to prevent excessive bleeding. Greyhounds (and possibly other sighthounds) tend to have naturally low platelet/thrombocyte counts (80-200 vs 150-400 thousand/mm3).

I have recently reviewed cases where dogs developed thrombocytopenia and sudden anemia subsequent to receiving leptospirosis-containing vaccines. A condition that appears similar to Evans Syndrome.

Reasons Why Your Pet’s Platelet Count Could Be High = Thrombocytosis

A higher than normal blood platelet count is quite rare. It is usually of no significance. Platelet counts are often a bit higher in immature pets. Ironically the same cancers (lymphoma and leukemias) that can cause thrombocyte numbers to decrease occasionally cause them to increase. Thrombocyte numbers are sometimes elevated in inflammation of the digestive tract and its accessory organs, the pancreas and liver (IBD, Pancreatitis). Platelet counts are occasionally high due to the high cortisol levels of Cushing's disease as well. Corticosteroid medication can have the same effect. Hypothyroidism has occasionally been associated with elevated thrombocyte levels. Very rarely, the bone marrow megakaryocytes that give rise to thrombocytes will themselves become cancerous - multiply and overproduce thrombocytes or the whole marrow system will go into "neoplastic overdrive" (polycythemia vera). When blood samples are hemolized, automated counting systems can mistake RBCs for platelets, giving falsely high counts.

Complementary Tests :

CBC/ WBC and standard blood chemistry panel, coagulation profile, M:E ratio, CT, bleeding time, Coombs test, ANA.
If the count was low in a lavender/blue top (EDTA) tube, repeat it in a green top (heparinized) tube.