Transfusions – Platelets

Platelet Transfusions: Do We Use Them?
Beth Davidow, DVM, DACVECC

Platelets are cytoplasmic fragments that circulate in the blood and are crucial for the initial formation of a clot. Platelets bind to exposed factors in damaged blood vessels and then bind to fibrinogen to form a clot. When activated, platelets also bind coagulation proteins to keep them at the site of the injury.

Platelets are short-lived and much more fragile than red blood cells or plasma proteins. Average platelet life span in the dog is 5-7 days. Platelets are quickly activated and no longer useable when refrigerated. Thus, platelet products for transfusion must be stored at room temperature, constantly agitated, and due to the risk of bacterial contamination, can only be stored for 5 days.

Thrombocytopenia, or low platelet count, is seen commonly in emergent veterinary patients but because of the complexity of storage and distribution, platelet transfusions are not often used in veterinary medicine.

Thrombocytopenia can be caused by decreased production, accelerated removal, increased consumption, loss, or dilution of platelets. The most common cause of thrombocytopenia in dogs is immune-mediated thrombocytopenia (ITP).3,4 Other causes include infectious diseases such as ehrlichiosis, bone marrow insults, neoplasia, disseminated intravascular coagulation (DIC) and blood loss.

Therapy with certain drugs can also result in thrombocytopenia. Azathioprine and chloramphenicol can lead to thrombocytopenia through a dose-related effect on the bone marrow.  In people, heparin can cause thrombocytopenia. Drug therapy with potentiated sulfonamides can also result in an immune-related thrombocytopenia in both dogs and people.

In the inherited macrothrombocytopenia of cavalier King Charles spaniels, a genetic mutation in the beta-tubulin protein of the microtubules is known to be the cause. Cavaliers affected with this condition can have platelet counts as low as 50,000/ul but often have no signs of bleeding. It is important to distinguish this condition from immune causes of platelet destruction as these animals do not require treatment.

In thrombocytopathic conditions, platelet numbers are normal but function is impaired. Acquired loss of platelet function is associated with medications and certain disease states. Non-steroidal anti-inflammatory drugs such as aspirin can decrease platelet function. Animals with both hepatic disease and uremia can also have platelet function issues.

Several hereditary thrombocytopathias have been identified in dogs and cats.8 Glanzmann thrombasthenia is a genetic disease that has been documented in great Pyrenees and otterhounds. Basset hounds can also have a genetic issue with platelet function.

 Indications for platelet transfusions

In humans, platelets are recommended for prophylaxis in any patient with a count less than 10,000/ul and in patients who need an invasive procedure with counts less than 50,000/ul. Platelets are recommended therapeutically in any actively bleeding patient with a count less than 20,000/ul. Platelets are also recommended in patients with drug or hereditary impairment of platelet function that need an invasive procedure. However, ITP is considered a unique situation due to the rapid clearance of any administered platelets. In addition, platelets in ITP are often younger and hyperfunctional so that bleeding may not occur until counts are extremely low. Platelet transfusions are usually NOT recommended for prophylaxis in this disease.

The risk of bleeding with thrombocytopenia is affected by the degree of anemia. A higher packed cell volume reduces the risk of bleeding. Thus, in cases of moderate thrombocytopenia and concurrent anemia, the risk of bleeding may be lessened with packed RBC transfusion alone.

The risk of bleeding from thrombocytopenia must be weighed against the risk of transfusion reaction especially when prophylactic platelet transfusions are considered. The reported rate of febrile reactions in people is 38% while 2% have a severe adverse reaction.

The standard fresh platelet concentrate dose of 1 U per 10 kg was derived to aim for a 40,000/ul increase in platelet count. Some investigators have looked at whether the interval between transfusions could be extended by using a higher dose initially. A meta-analysis in humans did show an increase in the interval between transfusions when a higher initial dose was used.

Currently Available Products

Fresh whole blood is the product most veterinarians use when platelets are needed. A 500 ml unit of fresh whole blood obtained from a canine donor is estimated to contain 7 X 1010   platelets. A dose of 10ml/kg of whole blood would be expected to raise the platelet count about 10,000/ul. The advantage of fresh whole blood is that no platelets are lost during separation. In addition, the platelets are less activated than in platelets obtained via centrifugation for concentrate. Fresh whole blood at room temperature is considered safe for use for 4-8 hours. As mentioned above, refrigeration of blood rapidly renders the platelets unusable for coagulation.

Fresh platelet concentrate has traditionally been made when initially processing whole blood. Whole blood is spun using a “soft” spin which separates the platelets into the plasma component, which is expressed into a separate bag. This plasma is then known as platelet rich plasma (PRP). The PRP is then spun again to create a platelet concentrate (PC), and the plasma is removed and stored as fresh frozen plasma. One unit of PC is the amount made from one unit (500 ml) of whole blood but will contain a lower amount of platelets. A recent study showed a maximum in vivo platelet recovery of 80%. The dose is normally calculated as 1 unit/ 10 kg. Fresh platelets must be stored in gas soluble bags at room temperature to remain active. They must also be constantly agitated and thus are kept on continuous rockers. Bacterial contamination is a concern at room temperature and storage is limited to 5 days. Because of the paucity of use and complexity of storage, the ACCES Blood Bank makes platelet rich plasma only on an as needed basis. This product cannot be easily shipped.

Frozen platelet concentrate is made by stabilizing apheresed platelets with 6% DMSO or with 2% DMSO and Thrombosol. In the original studies, canine platelet recovery after freezing with 6% DMSO at -80° F was shown to be 70% with a half-life of 2 days versus 3.5 days for fresh platelets.19 The platelets were shown to be effective in halting active bleeding in thrombocytopenic dogs. A more recent study comparing 6% DMSO to 2% DMSO and ThromboSol showed only 49% and 44% platelet recovery, respectively. Platelet half-life was confirmed to be about 2 days.18 The frozen product must be thawed at room temperature. This product can be obtained from Animal Blood Resources International (ABRI).

We are always happy to help if you have transfusion questions.