The SOLID Platelet Study

Background: Platelets are cell fragments in the blood that help it clot. Some people get very low platelet counts during a disease or treatment. Low platelet counts can cause severe bleeding. Some people are not helped by platelet transfusions at the standard transfusion rate. This is called platelet transfusion refractoriness (PTR). Researchers want to learn more about transfusing platelets so they can make transfusions more effective. Objectives: To study the effects of transfusing platelets more slowly than the standard rate. To obtain data to improve the effectiveness of platelet transfusions in people with PTR and decrease the risk of bleeding in some people. Eligibility: Adults ages 18-100 who have very low platelet counts requiring platelet transfusion, and have evidence of PTR Design: Participants will be screened with a review their recent NIH medical records. They will have blood drawn. Participants will have up to three 12-hour treatment blocks. They can have only one block per day. During each block, they will have 2 platelet transfusions in those 12 hours. One transfusion will take place over 1 hour (SHORT infusion). The other will take place over 4 hours (LONG infusion). Participants will be randomly put in 1 of 2 treatment groups. This will dictate whether they get the SHORT or LONG infusion first. Participants will have blood drawn: * When they enroll * Right before each transfusion * 2, 4, and 6 hours after each transfusion Each blood draw will consist of a complete blood count. Smaller tubes that require only small amounts of blood will be used to minimize the amount of blood drawn.