A platelet transfusion is an intravenous transfer of platelets collected from a donor or pooled from multiple donors. Platelets are small cell structures essential to the process of blood clotting and, therefore, important to maintain at certain levels within the body. A complete blood cell count (CBC) is a blood test that measures the number of the different types of cells circulating in the blood stream. When the results of a patient's CBC indicate that there are a lower than normal level of platelets, the condition is called thrombocytopenia. Depending on the cause, symptoms and severity of the patient's thrombocytopenia, a platelet transfusion may be considered as a treatment.

A platelet transfusion is performed by an intravenous drip into a vein in the arm or hand, an implanted central line, or a peripherally inserted central catheter (PICC) line. The donor platelets, which are stored in small bags, must be kept at cool temperatures. Each bag contains approximately 100 mL (3.4 oz) of the yellow fluid that includes the platelets. Most patients only require one bag, also called a unit, to be administered. Each unit takes about 15 to 30 minutes to be transfused.

Each transfused unit is typically comprised of platelets given by three to five blood donors or from those given by only one donor. When the blood of only one donor is used, the cells are known as apheresed platelets. These platelets are produced when the blood is processed through a machine, and the platelets are separated from the rest of the blood in a process called apheresis. Apheresed platelets are generally less likely to produce an adverse reaction in the recipient.

Platelet transfusions are often prescribed for those who are undergoing chemotherapy, bone marrow/stem cell transplant or organ transplant, and those with AIDS. Platelet transfusions are usually not given for thrombocytopenia secondary to the administration of the anti-clotting medication, Heparin. A platelet transfusion would also generally not be used to treat a patient with thrombotic thrombocytopenic purpura (TTP), a blood disorder that results in the formation of blood clots within the smallest blood vessels of the body.

Beyond a diagnosis that may indicate a need for a platelet transfusion, physicians also take other factors into consideration before prescribing this treatment. A patient with a platelet count of less than 10,000 requires a platelet transfusion, regardless of whether or not that patient shows any symptoms of thrombocytopenia. In general, a patient with a platelet count between 20,000 and 50,000 may be prescribed a platelet transfusion only if that patient is experiencing active bleeding. In addition, any patient with a platelet count under 50,000 may be prescribed a platelet transfusion, if he or she is undergoing surgery in the immediate future.