All HLA assignments, irrespective of the method, must comply with the current WHO Nomenclature Committee for Factors of the HLA System Report2 and Nomenclature for Factors of the HLA System, 20103 (and see EFI Standards D1.000–D1.320, inclusive). Examples of acceptable HLA assignments are as follows: HLA-B12, HLA-B44, HLA-B-44(12), HLA-B*44, HLA-B*44:09.
HLA may be serologically typed (to determine the phenotype) or typed by DNA molecular analysis. The term genotype is properly used to describe the genetic (DNA) constitution determined by the pattern of inheritance (EFI Standard D1.230).
HLA typing by DNA-based molecular techniques, which employ either DNA-based probes or primers, type for the presence or absence of sequence motifs. Kits using this technology are able to define the HLA alleles present in an individual to a variable level of resolution dependent on a number of factors. These include the number of probes or primers employed, the number of alleles defined for a given locus and the HLA alleles present in the individual. Although it is possible to achieve a high resolution or allele level typing using molecular methods, it is not a clinical requirement in transfusion practice. Therefore, patients and donors are typed to a low or medium level of resolution, and may present HLA typing results that include some ambiguity in interpretation.
Each serologically defined HLA antigenic specificity may be encoded by a number of different HLA alleles. Conversely many HLA alleles have no determined serologically defined antigen. Thus it is not always possible to assign a serological equivalent to each HLA allele.4 One consequence of this is that it is not practical to subject serological and DNA-based typing to the same standard as this would need to be unacceptably low (i.e. the lowest common denominator). Both techniques are in general use, each having specific advantages and disadvantages, and under these circumstances professional judgement together with the following guidelines should be used to deliver an appropriate standard of HLA typing.
Caution should therefore be exercised if an HLA type assigned using DNA-based molecular techniques is converted into a serological equivalent and such conversion must always be avoided with alleles for which the phenotype has not been unequivocally defined.