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titleWhy are there sometimes probabilities missing?

These donors don't have haplotype probabilities. This is usually the case when the phenotype could not be explained with the haplotypes given in the frequency set (inexplicable donor). Without haplotypical context, the programme gives only allele frequencies for the tested loci.matching engine is not able to provide match probabilities. 

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titleWhy is, for example, a donor with B*15:BPXE not shown as a potential match for a patient with B*15:03?

Since B*15:BPXE = B*15:03/61/74/103 a donor with this codes is a potential allele match for the patient. According to the official WMDA serology/DNA correspondence table, B*15:61 and B*15:74 have a serology of B15/B70 while B*15:03 is B72 and B*15:103 is B70. As a consequence a serology of B15 rules out B*15:03 and this donor is no longer potentially identical (on the allele level). Another explanation could be the limited length of donor lists.

Background: Unfortunately, for many donors serology was derived from DNA (by using the first field for the serological assignment) and vice versa (by appending “:XX” to the serological assignment) and often eventually both values are reported. In the case discussed, B*15:BPXE probably was translated back into B15 which is most likely wrong. This is a typical and (with certain registries) frequent case but there are many more unexpected DNA-serology correspondences that can give rise to exactly the same situation.

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