Platelet crossmatching. A direct approach to the selection of platelet transfusions for the alloimmunized thrombocytopenic patient.

Selection of platelets for alloimmunized, thrombocytopenic patients has traditionally been based on HLA matching. This approach is indirect and may not adequately recognize incompatibility between the recipient and the platelet donor. The authors evaluated the usefulness of directly showing donor-recipient compatibility by crossmatching the patient's serum with prospective platelet donors who were not preselected on the basis of their HLA type. Eleven alloimmunized patients were chosen for study, and crossmatching was done by a radiolabeled antiglobulin test. These patients had high levels of HLA alloantibody, and their unusual HLA types made the provision of HLA-matched platelets difficult. When the crossmatch was compatible, the mean one-hour corrected count increment was 18,379 +/- 4,670 (1 standard deviation), n = 22, and at 18-24 hours, 7,318 +/- 3,317. If the crossmatch was positive, the mean one-hour corrected increment was 2,536 +/- 3,057, and at 18-24 hours, 227 +/- 657, n = 16. There were two false negative crossmatches and one false positive crossmatch. One hundred forty-eight crossmatches were done to find 48 potential donors, who, by conventional selection using HLA matching, would not have been considered appropriate donors. These results show that successful platelet transfusions for alloimmunized thrombocytopenic patients can be prospectively selected by platelet crossmatching without the need of doing expensive HLA typing of a large population of platelet donors. Although it may be difficult to find compatible platelets for some patients with broadly reactive HLA antibodies, platelet crossmatching may detect compatible donors who are ordinarily excluded on the basis of their HLA phenotype.