Platelets are supplied in units that are equivalent to the number of platelets in one unit of whole blood (about 5.5 x 1010). These are obtained as single platelet units from random-donor whole blood units or as multiple platelet units from a single donor by means of platelet apheresis. Platelets are stored at room temperature up to 5 (sometimes 7) days. One single-donor unit ordinarily is expected to raise the platelet count 7,000-11,000/mm3 per square meter of body surface (equivalent to 5,000-10,000/mm3 [10-25 x 109/L] in an average-size adult). It has been suggested that platelets should be transfused as soon as possible after collection to retain maximum function and that infusions should be rapid (10 ml/min). Some also suggest a microaggregate filter (usually 40-µ size) if platelet storage has been more than 24 hours from time of collection. Platelet concentrates prepared by ordinary methods usually contain some plasma (about 50 ml/platelet unit) and some WBCs. Platelets from donors who have taken aspirin within 72 hours of donation may have deficient platelet function.

Platelet antigens. Platelets themselves contain various antigens, including ABO and HLA. Single-donor platelets can be transfused without typing (similar to random-donor platelets) or can be typed (usually ABO and HLA) for recipient compatibility before administration. Platelet ABO incompatibility usually has only a minor effect on donor platelet survival. HLA incompatibility may become a more serious problem. After repeated transfusions with random-donor nonmatched platelets about 50%-70% of patients (range, 10%-100%) become sensitized to platelet HLA antigens (or sometimes, to platelet-specific antigens), and these patients usually become refractory to additional platelet transfusions. Most can still be transfused with HLA-matched platelets. Siblings have the best chance of providing HLA-compatible blood, although nonsiblings often match adequately. Some institutions administer only HLA-matched platelets when long-term need for platelets is anticipated. However, the AABB currently recommends that patients receive non-HLA-matched platelets initially, with HLA-matched platelets reserved for those who become refractory. Some reports suggest that leukocyte depletion helps delay platelet sensitization. Some investigators perform a 1-hour and a 24-hour platelet count after transfusion. Low 1-hour recovery is said to suggest platelet antigen sensitization. When conditions that decrease platelet survival (e.g., fever, infection, or disseminated intravascular coagulation [DIC]) are present, the 1-hour count shows normal recovery but the 24-hour count is low. One report suggests that a platelet count 10 minutes after transfusion provides the same platelet values as the 1-hour count.

Indications for platelet transfusion. Platelet transfusion can be therapeutic or prophylactic. Therapeutic transfusions are indicated when severe acute bleeding occurs and the patient is severely thrombocytopenic (<50,000 platelets/mm3 or µL). When the patient has thrombocytopenia but has very minor bleeding or is not bleeding, the question of prophylactic platelet transfusion may arise. The decision is usually based on the degree of risk and the type of disorder being treated. Until 1993, most authorities considered patients to be high-risk if their platelet counts were less than 20,000/mm3 or µL (some used a cutoff value of 10,000/mm3); moderate-risk patients (transfusion only if clinically indicated) were those with counts of 20,000-50,000mm3; and low-risk patients included those with counts over 50,000mm3. Based on more recent studies, investigators are now proposing 5,000mm3 (or µL) as the threshold “trigger” value for prophylactic platelet transfusion (rather than 20,000 or even 10,000). The bleeding time has also been used as a guide to therapy; a bleeding time value of less than twice the upper limit of the reference range would not ordinarily need platelet transfusion. In patients with conditions that require multiple platelet transfusions over relatively long time periods, an additional consideration is the probability of developing antiplatelet antibodies that would interfere with therapy if actual bleeding developed later. In idiopathic thrombocytopenic purpura, antiplatelet antibodies that destroy donor platelets are already present, so that transfusion is useless unless the patient is actively bleeding. In drug-induced thrombocytopenia, transfusion is useless if the drug is still being given; after medication has been discontinued, transfusion can be helpful since a normal platelet count will usually return in about 1 week and transfused platelets survive about 1 week.

Platelet concentrates given to bone marrow transplant patients, severely immunodeficient or immunosuppressed patients, and blood relatives of the donor should have the platelet unit irradiated with at least 25 Gy to avoid graft-vs.-host disease.