Platelets

Description
Platelets are essential for the initial phase of hemostasis. Platelet concentrates also contain about 60mL of plasma and small numbers of red blood cells and leukocytes. Platelet units must be maintained at room temperature and agitated during storage.

Pooled random donor platelet concentrates are prepared from platelets that have been harvested by centrifuging units of whole blood. Up to 8 units of platelets, each from a separate donor, can be pooled into a single bag for transfusion. Platelets expire 4 hours after pooling. All units are from the same ABO type. If ABO compatible platelets are unavailable, ABO incompatible platelets can be substituted with very little risk. The usual adult dose is 4-6 units of pooled random donor platelets.

Apheresis platelets are collected from a single donor and are equivalent to ~4-6 pooled units. An apheresis platelet concentrate contains 200-400mL of plasma. They may be collected as a random unit (random apheresis platelets) or be obtained for a specific recipient from a family member or a volunteer HLA compatible “directed” donor. Apheresis platelets expire 4 hours after processing for release from the blood center unless incubated storage is available at the local hospital.

Indications

1. To prevent bleeding due to thrombocytopenia. The threshold of thrombocytopenia at which bleeding may occur will vary depending on the patient's clinical condition. In general, spontaneous bleeding does not occur until the platelet count falls below 5,000 - 10,000/μL. The recommended “trigger” for prophylactic platelet transfusions in patients undergoing chemotherapy or hematopoietic stem cell transplantation is <10,000/μL. Other coexisting clinical conditions may increase this "threshold".

2. In a bleeding patient a platelet count above 50,000 should be maintained. In a surgical patient, the necessary platelet count varies depending on the procedure. For most surgeries 30,000-50,000/μL will be adequate. For high risk procedures, such as neurologic or ophthalmologic surgeries, 100,000/μL is recommended.

3. Abnormal platelet function may be congenital, or due to medications, sepsis, malignancy, tissue trauma, obstetrical complications, extra corporeal circulation, or organ failure such as liver or kidney disease. Spontaneous bleeding may then occur at higher platelet counts. If platelet dysfunction is present, the patient with a disrupted vascular system (e.g. trauma or surgery) will require a higher platelet count to achieve hemostasis.

4. Family donor or HLA matched platelets are indicated when patients have become refractory to random donor platelet transfusions due to alloimmunization.

5. In several situations platelet transfusions may not be indicated unless there is significant bleeding. In autoimmune thrombocytopenias (e.g. ITP) transfusion increments are usually poor and platelet survival is short. Platelet transfusions may be contraindicated in patients with thrombotic thrombocytopenic purpura (TTP) unless there is clinically significant bleeding.

6. In pediatric patients, the usual platelet dose is 1 unit whole blood platelet per 10 kg child, or 5 mL/kg. A 50,000/ μL rise is expected.

Therapeutic Effect

*In a patient with a normal sized spleen and without platelet antibodies.

The survival of transfused platelets averages 3 to 5 days but will decrease if a consumptive process is present. Correction of a prolonged bleeding time in platelet dysfunction will depend on whether a condition exists that will affect the transfused platelets as well (e.g., antiplatelet agents, uremia).