What is the appropriate size of the worldwide cord blood inventory?
In spite HLA polymorphism distribution has some variations between major ethnic groups, in a US study by Beatty et al. (Human Immunology, 2000), 10 000 donors provided a 4/6 HLA-match for patient in every group using the conventional CB match criteria (low resolution for HLA-A,-B and high resolution for HLA-DRB1). This was also shown in our UK study where 10 000 randomly selected donors allowed finding a match for almost all patients (Querol, Haematologica, 2009). For better match level (5/6) a bigger inventory was required but enlarging size showed a progressive decrease on its efficiency. Thus, only 5000 units were required to find a donor for 50% of patients; but to achieve 65%, we then need to triplicate size (15 000), and growth up to 50 000, to match 80%. Further increase has only marginal benefit. We can then speculate that an optimal world size inventory be the addition of fractions of 5–15 000 donors for each major ethnic group considered, depending on their anticipated diversity, to at least provide options of finding a 5/6 donor for more than half of the patients but assuring a 4/6 match donor for almost any of them. In addition, inter-ethnic cross matching and other HLA favorable interactions like use of HLA homozygous for mixed ethnicities, and consideration of Non-Inherited Maternal Antigen (NIMA) match effect, if confirmed, would further increase donor availability. If we consider 23 major ethnic groups as proposed by the allele frequency net database (Gonzalez-Galarza, Nucleic Acid Research 2011), a worldwide CB inventory would be in the region of 300 000 units. This would suffice to provide an optimal compatible donor for any patient in need. To be effective, all units within the inventory should be larger than 1·5E9 total nucleated cells (20% of those collected) to serve patients up to 100 kg using the double CBT strategy (threshold 1·5E7 TNC/kg). A central world cord blood bank may achieve this equalitarian goal with €300 million budget and a proper world distribution and size of regional collection programmes.