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July, 1997 Darrell J. Triulzi, M.D., Medical Director, The Institute For Transfusion Medicine Introduction Platelet transfusions remain an integral part of the support of hematology/oncology and surgical patients. Appropriate use of this limited resource contributes to optimal patient care and minimizes health care costs.
Description Platelets used for transfusion can come from two sources: platelet concentrates derived from a unit of whole blood, called random donor platelet concentrates (RDP); or apheresis platelets obtained from a single donor by plateletpheresis. Each random donor platelet contains >5.5x1010 platelets in approximately 50 ml of plasma and is pooled in the required dose. A single donor platelet (SDP) obtained by apheresis contains the equivalent number of platelets in 6-8 units of RDP and contains approximately 200 - 300ml plasma.
Storage Both RDP and SDP can be stored up to five days at 20-24°C (room temperature). They must be continually agitated to prevent clumping. Storage is limited to five days due to the risk of bacterial contamination.
Compatibility Platelets are labeled with ABO and Rh type. ABO identical or compatible platelets are preferred. However, in adults ABO incompatible platelets may be uses because the amount of plasma is rarely of clinical concern. Crossmatching is not required because platelets contain only small numbers of contaminating red cells (2-5 ml). However this number of red cells is capable of causing immunization to the D antigen. Thus Rh compatibility is required in Rh negative children and women of child bearing age to prevent the formation of anti-D and potentially hemolytic disease of the newborn.
Dosage The recommended dose of RDP is 1unit/10kg body weight.1,2 This dosing schedule can be used for infants, children, and adults. The expected increment in platelet count is 5-10,000/ul per unit of platelet transfused. The smaller the patient the larger the relative dose. Thus an infant or small child may increment 25-50,000/ul per unit of RDP.
For adults, 1 SDP is a therapeutic dose. For children, an SDP may contain an excessive dose of platelets. In these situations the SDP may be split or aliquoted. The expected platelet count increment from an SDP in an adult is 25 - 50,000/ul.
Indications Platelet transfusions are indicated to treat or prevent bleeding in patients with clinically significant deficiencies in platelet number or function. The indication for prophylactic platelets to prevent bleeding in patients with severe thrombocytopenia is an evolving area. Recent studies3,4 and several years of experience have shown that the previously recommended "trigger" of 20,000/ul is not required for many patients since the risk of significant spontaneous bleeding in an otherwise stable patient does not occur until the platelet count is <10,000/ul. Patients with fever, sepsis, coagulopathy, or anatomic bleeding (i.e. severe mucocytis) may require higher platelet levels. In the bleeding patient or a patient who is undergoing an invasive procedure platelet transfusions are recommended for counts of <50,000/ul. Patients with a coagulopathy in addition to thrombocytopenia may require transfusion at higher levels. Prophylaxis: <10,000/ul (stable patient) Bleeding patient: <50,000/ul Invasive procedure: <50,000/ul Deficiencies in platelet function may be observed in the absence of thrombocytopenia. While some causes of platelet dysfunction such as aspirin induced can be treated with platelet transfusion, others such as uremic bleeding are not appropriately treated with platelet transfusion. Consultation is recommended for management of patients with platelet dysfunction.
Complications / Risks Platelet transfusions are accompanied by fever-chill reactions in 1% of all transfusions but in as many as 30% of transfusions in multitransfused patients. These reactions occur due to white cells contaminating the platelet component or to cytokines released by the white cells into the platelet supernatant during storage. The reactions can be mitigated by using leukoreduced platelet components and/or using platelets with reduced storage time i.e. 3 days. Allergic reactions occur in 1% of platelet transfusions and are generally mild and easily treated with antihistamines. Rare reactions to platelets include: septic reactions due to bacterial contamination, graft vs. host disease, and ARDS (transfusion related acute lung injury). The risk of viral transmission is the same as that for any unit of
blood. However, since RDP are pooled they typically contain 6-8 donor exposures Vs 1 for
an SDP. Fortunately the risk of viral transmission has become so small (see table) that
the difference in donor exposures between a pool and an SDP is of little concern in the
patients who most frequently require platelet transfusions; stem cell transplant
recipients and older patients undergoing cardiac surgery. RISK ESTIMATES FOR TRANSFUSION TRANSMITTED VIRUSES PITTSBURGH VS U.S. Risk per tested unit -1996 / 1997
* U.S. risks from Schreiber GB et al. NEJM 1996;334:1685 ** Risks based on Central Blood Bank donor prevalence statistics
Future considerations Research is underway to develop virally inactivated platelets and platelet substitutes derived either from synthetic thrombogenic materials or from lyophilized platelet membranes. These products are still several years away. Much sooner we will see the availability of thrombopoietin to stimulate the patients own platelet production. Clinical trials with thrombopoietin are ongoing.
References 1. NIH Consensus Conference-Platelet Transfusion Therapy. JAMA 1987;257:1777. 2. Practice parameter for the use of FFP, cryoprecipitate,and platelets. 3. Beutler E. Platelet transfusions: The 20,000/ul trigger. Blood 1993;81:1411. 4. Heckman KD et al. Randomized study of prophylactic platelet transfusion threshold during induction therapy for ALL: 10,000/ul Vs 20,000/ul. J Clin Onc 1997;15:1143.
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