existing data reveals that to ensure a 90 percent probability of a unit with a match of 4/6 HLAs and with a minimum cell dose of 2.5 × 107 total nucleated cells per kilogram of body weight (TNC/kg), at least 100,000 units are required. Increasing the minimum cell dose to 3.0 × 107 TNC/kg increases the minimum number of units needed to 200,000. This preliminary analysis reveals that the relatively small changes to the minimum system requirements that are needed to maximize efficiency can lead to dramatic increases in the inventory size. In either case, substantial increases to the existing usable inventory are required, as are new criteria for what constitutes an acceptable cord blood unit. (For more information on HLA typing, see Appendix F.)
The likelihood of finding a 4/6 or a 5/6 HLA match greatly exceeds the likelihood of finding a 6/6 HLA match. Finding a match is even more difficult within the African-American population, for which the likelihood of finding a 5/6 HLA match is only 50 percent, whereas the likelihood is 80 percent likelihood for the population as a whole (Howard et al., 2005) (see Appendix E). In addition, individuals of mixed race make up an increasing proportion of the population and may have particular difficulties in finding HLA-matched donors. A 3/6 HLA match can always be found, but no published data support the routine use of a 3/6 HLA match in clinical cord blood transplantation.
The transplant center must weigh several different competing interests when it chooses a unit for a patient in need. Often the “best available” unit can get lost in pursuit of the “ideal” unit.
Different approaches to unit selection also exist, depending on differing transplant center philosophies and emerging information regarding outcomes. The level of HLA match required, the cell dose, and the particular interplay between these two measures are under constant scrutiny by transplant physicians and banks. Using data provided by the New York Blood Center, the National Marrow Donor Program, and the National Heart, Lung and Blood Institute, the committee conducted an outcomes analysis to better understand these measures.
In an analysis of data for first transplants, it became very clear that the rate of survival after the transplantation of cord blood units with low cell doses (<2.5 × 107 TNC/kg) and matched for 4/6 or 5/6 HLAs is substantially lower than that after the transplantation of units matched for 6/6 HLAs with low cell doses. As the cell dose increases, the adverse effects of