For patients with certain blood cancers or other diseases, hematopoietic stem cell transplants (HSCT) are potentially lifesaving therapies. While the majority of patients are able to find a suitable, unrelated donor, the likelihood of finding a match varies quite dramatically based on a patient’s race and ethnicity. In a recent New England Journal of Medicine article, researchers from the National Marrow Donor Program (NMDP) tried to measure the odds of finding a suitable donor in the NMDP-operated Be the Match® Registry.
Led by Loren Gragert, BS, BA, at the NMDP,the researchers reported that Caucasians had the greatest likelihood (75%) of finding an optimal HLA-matched adult donor, while African Americans had less than a 20 percent chance. The likelihood of an ideal match (8/8 allele level match, or matching at a high resolution for HLA-A, -B, -C, and -DRB1) for other ethnic/racial groups fell in between: 41 percent for Chinese patients, 33 percent for South Asians, 37 percent for Mexicans, and 52 percent for Native Americans.
Across the board, a drop to a 7/8 match requirement increased the likelihood of finding a donor, and the authors pointed out that “patients belonging to groups with high levels of genetic admixture, such as Hispanic groups, are more likely to have donors identified from outside their group.”
Age also played a major role in finding an ideal, 6/6 cord-blood match: A patient of white European descent under 20 years of age had a 38 percent probability of finding an ideal cord blood match, but that probability dropped to 16 percent when the patient was older than 20. For African-American patients, the corresponding, age-based chances were 6 percent and 2 percent. This age-based relationship was likely due to the fact that “most units have sufficient cells to provide an adequate cell dose for patients who weigh less than 50 kg [110 lbs],” the authors explained.Going for a lower match, though, such as 4/6, meant that almost all younger patients would have a cord blood match with adequate cell dose.
“For both graft sources, the racial and ethnic group of the patient strongly influences the likelihood of having a suitable graft identified,” Mr. Gragert told ASH Clinical News. “The results suggest that almost all patients likely to benefit from HSCT will have a donor. However, many will not have an optimal adult donor – that is, a donor who is matched at high resolution at HLA-A, HLA-B, HLA-C, and HLA-DRB1, or an 8/8.”
The authors reached these conclusions by creating population-based genetic models for 21 racial and ethnic groups to predict the likelihood of identifying a suitable adult donor or cord blood unit in each group. They applied these models to the HLA genotypes and cord blood units from about 10 million HLA adult donors and nearly 187,000 donors in the NMDP registry.
According to these data, most U.S. patients who are likely to benefit from HSCT will find a donor. The growth of the NMDP registry is also encouraging for patients seeking donors: NMDP added a little more than a million donors in 2012 and is expecting a cumulative growth rate of 9 percent through 2017.
The investigators issued one note of caution when interpreting these data for transplant search: the type of donor or graft identified will fluctuate based on the search strategy used by the transplant center. A search strategy in which cord blood units are given priority over optimal, or near optimal, HLA-matched adult donors boosted the chances of making a match across all ethnic and racial groups.
Gragert L, Eapen M, Williams E, et al. HLA match likelihoods for hematopoietic stem-cell grafts in the U.S. registry. N Engl J Med.2014;371:339-48.