About 90 registries and cord blood banks of HLA typed potential donors have been established in 41 countries and include over 9 million individuals/units (25, 79, 80). These registries/banks range in size from 42 units in a small cord blood bank to over 5 million adult volunteer donors in one registry. The HLA phenotypes available worldwide are summarized within the database of Bone Marrow Donors Worldwide (BMDW; http://www.bmdw.leidenuniv.nl). In 2002, worldwide registries received over 77,000 preliminary search requests and processed over 29,000 activated searches. There is, for example, an average of 7,000 patients searching the world’s largest registry, the NMDP, at any given time.
While an unrelated donor search usually begins within the country of origin of the patient, searches failing to identify a donor can extend to foreign registries/banks. Of the almost 5,900 unrelated transplants worldwide, about one-third involved stem cells from a donor in another country. For the NMDP, approximately equal numbers of hematopoietic stem cells were provided from overseas donors to U.S. patients (1,700) as were provided by U.S. donors for patients abroad (1,946). An international voluntary organization of registries worldwide, the World Marrow Donor Association (WMDA; http://www.worldmarrow.org), has published policies and procedures for these international exchanges and WMDA working groups such as the Information Technology Working Group, are focused on standardizing data elements, forms and processes for international exchanges (18, 22, 30, 63).
There is no single accepted definition of a “match” in regards to a specific transplant donor. The match can be at allele level or at antigen level. For example, a patient and donor sharing the alleles at the DRB1 locus, DRB1*0101, DRB1*0302, are said to be allele matched. Sharing DRB1*01, DRB1*03 (low resolution DNA typing) or DR1,DR3 (serologic assignment) would be defined as an antigen match. At the lower level of resolution, the donor and recipient are actually potentially matched at the allele level since they each carry one of 11 alleles of DRB1*01 and one of 25 alleles of DRB1*03. The probability of the two individuals carrying the same alleles depends on the frequency of the alleles in the racial/ethnic group of each individual. For example, if the patient carries DRB1*0302, the probability of a DRB1*03 positive African American volunteer carrying the same allele is about 46 percent, while the probability of a DRB1*03 positive white individual carrying the allele is 0 percent (see Table F-2).